D3MIRT ModelingThe D3mirt analysis is based on descriptive
multidimensional item response theory (DMIRT; Reckase, 2009, 1985;
Reckase & McKinley, 1991) and can be used to analyze dichotomous and
polytomous items to measure latent traits (denoted \(\theta\)) in a three-dimensional ability
space. The method is foremost visual and illustrates item
characteristics with the help of vector geometry.
In DMIRT analysis, it is assumed that items in a multidimensional ability space can measure single or multiple latent traits (Reckase, 2009, 1985). This assumption is realized in the compensatory model, i.e., a type of measurement model that uses linear combinations of \(\theta\)-values for ability assessment (Reckase, 2009). DMIRT builds on the results from the former model and seeks to maximize item discrimination in the latent space by relaxing the assumption of unidimensionality. The method is descriptive because the results describe both the strength of discrimination of the items and the extent to which the items are unidimensional, i.e., that the items discriminate on one dimension only, or are within-multidimensional, i.e., that the maximum item discrimination is reached when the item measures more than one dimension.
Regarding vector orientation, the angle of the vector arrows indicates what traits, located along the axes in the model, an item measures (Reckase, 2009, 1985; Reckase & McKinley, 1991). For instance, in a two-dimensional space, an item is unidimensional if its item vector arrow is at \(0°\) with respect to one of the axes in the model and \(90°\) with respect to the other. Such an item describes a single trait only. In contrast, an item is perfectly within-multidimensional if its item vector arrow is oriented at \(45°\) in relation to the axes in the model. Such an item describes both traits in the model equally well. The same criteria are extended to the \(n\)-dimensional case.
The DMIRT approach uses two types of item models depending on item type. If dichotomous items are used, the analysis is based on the multidimensional two-parameter logistic model (M2PL) (McKinley & Reckase, 1983; Reckase, 1985). If polytomous items are used, the analysis is extended to the multidimensional two-parameter graded response model (MGRM; Muraki & Carlson, 1995). The method is, therefore, limited to items that fit these item models.
The estimation begins by first fitting and extracting the loadings \(a\) and difficulty parameters \(d\) from a compensatory model. Next, the DMIRT estimation takes the former estimates and computes the multidimensional discrimination (\(MDISC\)) parameter and the multidimensional difficulty (\(MDIFF\)) parameter used to locate the items in a multidimensional vector space.
The \(MDIFF\) is interpreted similarly to the difficulty parameter in the unidimensional model. It shows the level of ability required for a higher or correct response, defined as the distance from the origin. Note that if polytomous items are used, such as Likert items, the items will be represented by multiple vector arrows, one for each response function. The \(MDIFF\) will then show an item’s multidimensional range of difficulty as located in the latent trait space.
The \(MDISC\) shows the highest level of discrimination an item can achieve in the multidimensional latent model. It is, therefore, a global item characteristic referring only to the discrimination achieved in the specific direction of the item in the latent space. The \(MDISC\) score is visualized in the graphical output by scaling the length of the vector arrows (longer arrows indicate higher discrimination and vice versa), which in turn describe the domain of item response functions.
In D3mirtit also possible to use constructs in the
analysis. Constructs, in this context, refer to the assumption that a
subset of items or a particular orientation in the model space can
measure a higher-order latent variable. This is implemented in
D3mirt as optional vectors whose orientation is either
calculated as the average direction of a subset of items (from all items
down to a single item) or indicated by spherical coordinates. A
construct vector will, therefore, point in the direction of the maximum
slope of what we might think of as an imaginary item response function
indicated by the items or coordinates chosen by the user based on
exploratory or theoretical reasons.
If constructs are used, the output will include reporting the directional discrimination (\(DDISC\)) parameter that shows how well the items discriminate, assuming that they measure one of the constructs used in the analysis. That is, while the \(MDISC\) represents the maximum level of discrimination in the model, the \(DDISC\) represents the local discrimination that makes it possible to compare item discrimination in a specific direction set by the constructs. The constructs are, therefore, like unidimensional models nested in the multidimensional latent space and are visually represented with construct vector arrows scaled to an arbitrary length.
The package includes the following main functions.
modid(): D3mirt Model IdentificationD3mirt(): 3D DMIRT Model Estimationplot(): Graphical Output for D3mirt()You can install the D3mirt package from CRAN, or try the
development version of the package, by using the following codes for
R.
# Install from CRAN depository
install.packages('D3mirt')
# Install the development version from GitHub
# install.packages("devtools")
# To include package vignette in the installation, add: build_vignettes = TRUE
devtools::install_github("ForsbergPyschometrics/D3mirt")In what follows, the D3mirt procedure, including the
main functions and some of the more essential arguments, will be briefly
described using the built-in data set “anes0809offwaves”. The data set
(\(N = 1046, M_{age} = 51.33, SD = 14.56,
57\%\) Female) is a subset from the American National Election
Survey (ANES) from the 2008-2009 Panel Study Off Wave Questionnaires,
December 2009 (DeBell, et al, 2010; https://electionstudies.org/data-center/2008-2009-panel-study/).
All items measure moral preferences and are positively scored of Likert
type, ranging from 1 = Strongly Disagree to 6 = Strongly
Agree. Demographic variables include age and gender
(male/female).
The D3mirt approach largely consists of the following three steps:
Please see the vignette included in the package for more details on
the D3mirt package, including extended examples of analysis
and functions.
As a first step in the analysis, the three-dimensional compensatory model must be identified (Reckase, 2009). In the three-dimensional case, this implies identifying the \(x\) and \(y\)-axis in the DMIRT model by selecting two items from the item set. The first item should not load on the second and third axes (\(y\) and \(z\)-axes), while the second item should not load on the third axis (\(z\)). Consequently, if the model is not known beforehand, it is necessary to explore the data with exploratory factor analysis (EFA), preferably with the help of the EFA methods suitable for DMIRT.
The modid() function was designed to help with this step
in the process. In brief, the function first performs an EFA based on
multidimensional item response theory. It then selects items so that the
strongest loading item, from the strongest factor, is always parallel
with the x-axis, and the remaining items follow thereafter. This helps
make the result maximally interpretable while also avoiding imposing an
unempirical structure on the data.
Note that the EFA is only used to find model identification items that meet the necessary DMIRT model specification requirements. The EFA model itself is discarded after this step in the procedure. This implies that the rotation method is less crucial, and the user is encouraged to try different rotation methods and compare the results.
Begin by loading the item data. Note that all outputs are available as ready-made package files that can be loaded directly into the R session.
# Load Package
library(D3mirt)
# Load data
data("anes0809offwaves")
x <- anes0809offwaves
x <- x[, 3:22] # Remove columns for age and genderThe modid() can take in raw item data or a data frame
with item factor loadings. In the default mode
(efa = TRUE), using raw data, the function performs an EFA
using with the help of the mirt() function, from the
mirt package (Chalmers, 2012), with three factors as the
default (factors = 3). After sorting the results following
the model identification procedure, the function finishes by presenting
the model identification items. The function throws an error if no items
can be located. Note that it is possible to use available factor
loadings assigned to a data frame in the function call. This can be done
by setting efa to FALSE, which allows the
function to skip the EFA and jump directly to the model identification
procedure.
The output consists of an \(S3\)
object of class modid containing data frames with model
identification items, order of factor strength (based on sum of
squares), and item factor loadings. The function has two arguments: the
lower and upper bound. In brief, the lower bound
increases the item pool used in the procedure, while the upper bound
acts as a filter that removes items that do not meet the necessary
statistical requirements. This implies that the upper bound should not
generally be manipulated.
# Optional: Load the EFA data for this example directly from the package file
load(system.file("extdata/id.Rdata", package = "D3mirt"))# Call to modid() with x, containing factors scores from the EFA
# Observe that the efa argument is set to false
id <- modid(x)summary(id)
#>
#> modid: 20 items and 3 factors
#>
#> Model identification items:
#> Item 1 W7Q3
#> Item 2 W7Q20
#>
#> Item.1 ABS
#> W7Q3 0.8547 0.0174
#> W7Q5 0.8199 0.0648
#> W7Q1 0.7589 0.0772
#> W7Q10 0.7239 0.0854
#>
#> Item.2 ABS
#> W7Q20 0.7723 0.0465
#> W7Q19 0.6436 0.0526
#> W7Q18 0.6777 0.0782
#>
#> SS Loadings
#> F2 5.3505
#> F1 2.1127
#> F3 1.6744
#>
#> F2 F1 F3
#> W7Q1 0.7589 0.0407 -0.0365
#> W7Q2 0.8901 -0.0263 -0.0838
#> W7Q3 0.8547 -0.0096 -0.0078
#> W7Q4 0.6628 0.0272 0.1053
#> W7Q5 0.8199 -0.0390 -0.0258
#> W7Q6 0.6654 0.0525 0.1054
#> W7Q7 0.5604 -0.0148 0.2087
#> W7Q8 0.5731 0.0390 0.1966
#> W7Q9 0.6151 0.0697 0.0918
#> W7Q10 0.7239 0.0371 -0.0483
#> W7Q11 0.2085 0.0959 0.5488
#> W7Q12 0.0755 -0.0853 0.5559
#> W7Q13 -0.0176 -0.0153 0.7654
#> W7Q14 -0.0407 0.1439 0.5629
#> W7Q15 0.1087 0.4556 -0.1111
#> W7Q16 0.1759 0.2100 0.1152
#> W7Q17 0.2160 0.5816 0.0261
#> W7Q18 -0.0560 0.6777 -0.0782
#> W7Q19 0.0589 0.6436 0.0526
#> W7Q20 -0.0735 0.7723 0.0465The summary() function prints the number of items and
the number of factors used in the analysis together with the suggested
model identification items. As can be seen, the items suggested by
modid() are the items “W7Q3” and “W7Q20”. The output also
includes data frames that hold all the model identification items
(Item.1...Item.n) selected by modid() together
with the items’ absolute sum score (ABS), one frame for the
sum of squares for factors sorted in descending order, and one frame for
item factor loadings.
The order of the factors follows the model identification items so that item 1 comes from the strongest factor (sorted highest up), item 2 from the second strongest (sorted second), and so on. The absolute sum scores indicate statistical fit to the structural assumptions of the DMIRT model, and the items are sorted with the lowest absolute sum score highest up. Thus, the top items are the items that best meet the necessary statistical requirements for the model identification. For a three-dimensional model, this implies that the item highest up in the first data frame should be used to identify the \(x\)-axis and the item highest up in the second data frame should be used to identify the \(y\)-axis, and so on. See the package vignette for more on the model identification procedure (e.g., troubleshooting, criteria, or limitations).
The D3mirt() function holds two built-in models. The
first model is the default model, i.e., the three-dimensional
compensatory model, and the latter is the orthogonal model,
i.e., a restricted model in which the assumption of
within-multidimensionality is removed. In this context,
orthogonal refers to the strict perpendicular orientation of
the items in the latent space. The choice of model depends on the vector
used in the modid argument when calling
D3mirt() (see examples below).
Calling D3mirt() returns an \(S3\) object of class D3mirt
with lists containing \(a\) and \(d\) from the compensatory model, and the
\(MDISC\), and \(MDIFF\) parameters, direction cosines, and
spherical coordinates for the item vectors from the DMIRT model.
Regarding the latter, spherical coordinates are represented by \(\theta\) and \(\phi\). The \(\theta\) coordinate is the positive or
negative angle in degrees, starting from the \(x\)-axis, of the vector projections from
the vector arrows in the \(xz\)-plane
up to \(\pm 180°\). Note that the \(\theta\) angle is oriented following the
positive pole of the \(x\) and \(z\) axis so that the angle increases
clockwise in the graphical output. The \(\phi\) coordinate is the positive angle in
degrees from the \(y\)-axis and the
vectors. Note, the \(\rho\) coordinate
from the spherical coordinate system is in DMIRT represented by the
\(MDIFF\), and so is reported
separately.
Constructs can be included in the analysis by creating one or more
nested lists that indicate what items belong to what construct (see
example below) and using it in the con.items argument. From
this, the D3mirt() function finds the average direction of
the subset of items contained in each nested list by adding and
normalizing the direction cosines for the items and scaling the
construct direction vector to an arbitrary length (the length can be
adjusted by the user) so that the arrows can be seen when plotting. To
use spherical coordinates, the user must create a nested list containing
spherical coordinate pairs in the same format as the item indicators.
For example: con <- list(c(0, 45)), and so on. The
con vector, in the latter case, gives one construct vector
located \(45^{\circ}\) strictly above
the \(x\)-axis in the latent space.
If constructs are used, the function also returns construct direction cosines, spherical coordinates for the construct vector arrows, and \(DDISC\) parameters (one index per construct).
The summary() function presents the DMIRT estimates. The
constructs included below were grouped based on exploratory reasons,
i.e., because these items cluster in the model (observable in the
graphical output below).
# Optional: Load the mod3 data as a data frame directly from the package file
load(system.file("extdata/mod3.Rdata", package = "D3mirt"))# Call to D3mirt(), including optional nested lists for three constructs
# Item W7Q16 is not included in any construct because of model violations
# The model violations for the item can be seen when plotting the model
con <- list(c(1:10),
c(11:14),
c(15:20))
mod3 <- D3mirt(x, modid = c("W7Q3", "W7Q20"), con.items = con)summary(mod3)
#>
#> D3mirt: 20 items and 5 levels of difficulty
#>
#> Compensatory model
#> Model identification items: W7Q3, W7Q20
#>
#> Constructs
#> Item vector 1: W7Q1, W7Q2, W7Q3, W7Q4, W7Q5, W7Q6, W7Q7, W7Q8, W7Q9, W7Q10
#> Item vector 2: W7Q11, W7Q12, W7Q13, W7Q14
#> Item vector 3: W7Q15, W7Q16, W7Q17, W7Q18, W7Q19, W7Q20
#>
#>
#> a1 a2 a3 d1 d2 d3 d4 d5
#> W7Q1 2.0297 0.1645 -0.1227 8.0868 7.0641 5.9876 3.2016 -0.4834
#> W7Q2 2.6215 -0.0025 -0.2576 9.2884 6.6186 4.5103 1.6650 -2.4437
#> W7Q3 2.7932 0.0000 0.0000 10.4884 7.5922 5.6797 2.7181 -1.1790
#> W7Q4 1.9043 0.1877 0.1502 7.3749 6.0464 4.9812 2.4830 -1.1144
#> W7Q5 2.2427 -0.0285 -0.0836 8.4293 6.6722 4.9055 1.8256 -1.8316
#> W7Q6 2.0020 0.2392 0.1578 8.0687 6.3578 4.9521 2.3301 -1.0187
#> W7Q7 1.6284 0.1036 0.3600 6.0180 4.8976 3.6909 1.6326 -1.3483
#> W7Q8 1.7773 0.2254 0.3536 6.9174 5.1824 3.7663 1.4845 -1.8331
#> W7Q9 1.7197 0.2495 0.1286 7.5588 4.9756 3.3649 0.9344 -2.2093
#> W7Q10 1.7697 0.1274 -0.1404 8.3641 5.7399 4.2865 1.9648 -0.6641
#> W7Q11 1.4237 0.4680 1.0449 6.2204 4.6938 3.5443 1.1923 -1.8579
#> W7Q12 0.7601 0.0413 0.9370 4.1361 2.8772 2.3420 1.1791 -0.4240
#> W7Q13 1.1265 0.2914 1.6908 5.8838 4.3950 3.4385 1.8931 -0.6004
#> W7Q14 0.7444 0.4832 0.9787 5.3893 3.9334 3.0259 0.8144 -1.5869
#> W7Q15 0.4551 0.7871 -0.1607 4.3208 3.0545 2.3970 0.9187 -0.9705
#> W7Q16 0.6236 0.4141 0.1798 3.7249 2.0305 1.1658 -0.0612 -1.8084
#> W7Q17 1.1891 1.3414 0.0561 6.9016 5.8026 4.9348 2.7917 -0.0040
#> W7Q18 0.4106 1.3543 -0.1372 3.7838 2.0986 1.4183 0.1829 -1.9855
#> W7Q19 0.8578 1.4100 0.2278 4.4979 2.6484 1.6731 0.3741 -1.9966
#> W7Q20 0.7355 1.9066 0.0000 4.6376 2.3632 1.2791 -0.3430 -2.9188
#>
#> MDISC MDIFF1 MDIFF2 MDIFF3 MDIFF4 MDIFF5
#> W7Q1 2.0401 -3.9640 -3.4627 -2.9350 -1.5693 0.2370
#> W7Q2 2.6341 -3.5262 -2.5127 -1.7123 -0.6321 0.9277
#> W7Q3 2.7932 -3.7550 -2.7181 -2.0334 -0.9731 0.4221
#> W7Q4 1.9194 -3.8423 -3.1502 -2.5952 -1.2936 0.5806
#> W7Q5 2.2444 -3.7557 -2.9728 -2.1857 -0.8134 0.8160
#> W7Q6 2.0224 -3.9897 -3.1437 -2.4486 -1.1521 0.5037
#> W7Q7 1.6710 -3.6015 -2.9310 -2.2088 -0.9771 0.8069
#> W7Q8 1.8261 -3.7881 -2.8380 -2.0625 -0.8130 1.0038
#> W7Q9 1.7425 -4.3380 -2.8555 -1.9311 -0.5362 1.2679
#> W7Q10 1.7798 -4.6995 -3.2251 -2.4084 -1.1040 0.3731
#> W7Q11 1.8269 -3.4048 -2.5692 -1.9400 -0.6526 1.0170
#> W7Q12 1.2073 -3.4260 -2.3832 -1.9399 -0.9766 0.3512
#> W7Q13 2.0525 -2.8667 -2.1413 -1.6753 -0.9223 0.2925
#> W7Q14 1.3211 -4.0792 -2.9772 -2.2903 -0.6164 1.2011
#> W7Q15 0.9232 -4.6800 -3.3085 -2.5963 -0.9951 1.0512
#> W7Q16 0.7699 -4.8384 -2.6375 -1.5143 0.0795 2.3490
#> W7Q17 1.7935 -3.8482 -3.2354 -2.7515 -1.5566 0.0022
#> W7Q18 1.4218 -2.6613 -1.4761 -0.9976 -0.1286 1.3965
#> W7Q19 1.6661 -2.6997 -1.5896 -1.0042 -0.2245 1.1984
#> W7Q20 2.0436 -2.2694 -1.1564 -0.6259 0.1678 1.4283
#>
#> D.Cos X D.Cos Y D.Cos Z Theta Phi
#> W7Q1 0.9949 0.0806 -0.0601 -3.4590 85.3756
#> W7Q2 0.9952 -0.0010 -0.0978 -5.6133 90.0554
#> W7Q3 1.0000 0.0000 0.0000 0.0000 90.0000
#> W7Q4 0.9921 0.0978 0.0783 4.5098 84.3885
#> W7Q5 0.9992 -0.0127 -0.0372 -2.1341 90.7284
#> W7Q6 0.9899 0.1183 0.0780 4.5068 83.2064
#> W7Q7 0.9745 0.0620 0.2154 12.4657 86.4441
#> W7Q8 0.9733 0.1235 0.1936 11.2521 82.9082
#> W7Q9 0.9869 0.1432 0.0738 4.2769 81.7671
#> W7Q10 0.9943 0.0716 -0.0789 -4.5357 85.8965
#> W7Q11 0.7793 0.2561 0.5719 36.2765 75.1588
#> W7Q12 0.6296 0.0342 0.7761 50.9493 88.0399
#> W7Q13 0.5488 0.1420 0.8238 56.3262 81.8384
#> W7Q14 0.5634 0.3657 0.7408 52.7448 68.5472
#> W7Q15 0.4929 0.8525 -0.1740 -19.4451 31.5154
#> W7Q16 0.8101 0.5378 0.2335 16.0800 57.4627
#> W7Q17 0.6630 0.7479 0.0313 2.7031 41.5881
#> W7Q18 0.2888 0.9525 -0.0965 -18.4742 17.7264
#> W7Q19 0.5148 0.8463 0.1367 14.8712 32.1870
#> W7Q20 0.3599 0.9330 0.0000 0.0000 21.0939
#>
#> C.Cos X C.Cos Y C.Cos Z Theta Phi
#> C1 0.9970 0.0688 0.0368 2.1119 86.0548
#> C2 0.6409 0.2029 0.7404 49.1207 78.2961
#> C3 0.5405 0.8411 0.0226 2.3974 32.7476
#>
#> DDISC1 DDISC2 DDISC3
#> W7Q1 2.0304 1.2433 1.2326
#> W7Q2 2.6038 1.4887 1.4088
#> W7Q3 2.7847 1.7901 1.5096
#> W7Q4 1.9169 1.3697 1.1905
#> W7Q5 2.2308 1.3696 1.1862
#> W7Q6 2.0181 1.4484 1.2868
#> W7Q7 1.6438 1.3312 0.9754
#> W7Q8 1.8004 1.4465 1.1582
#> W7Q9 1.7364 1.2479 1.1422
#> W7Q10 1.7679 1.0560 1.0604
#> W7Q11 1.4900 1.7809 1.1867
#> W7Q12 0.7951 1.1892 0.4668
#> W7Q13 1.2053 2.0328 0.8922
#> W7Q14 0.8113 1.2997 0.8308
#> W7Q15 0.5019 0.3324 0.9043
#> W7Q16 0.6568 0.6168 0.6894
#> W7Q17 1.2799 1.0758 1.7722
#> W7Q18 0.4975 0.4363 1.3578
#> W7Q19 0.9605 1.0044 1.6547
#> W7Q20 0.8644 0.8581 2.0011The D3mirt() function prints a short report containing
the number of items used and the number of difficulty levels when the
estimation is done. As can be seen, when construct vectors are used, the
function also prints the number of construct vectors and the names of
the items included in each construct. Next, the factor loadings and the
difficulty parameters from the compensatory model are reported in data
frames, followed by all necessary DMIRT estimates.
Regarding the orthogonal model, the function call must include nested lists indicating which items should be constrained to either of the axes. Below is an example in which items \(1\) to \(10\) are constrained to the \(x\)-axes, items \(15\) to \(19\) to the \(y\)-axis, and items \(11\) to \(14\) to the \(z\)-axis. Note that fitting the orthogonal model often requires removing misfitting items. Otherwise, the orthogonal model most likely will amplify the misfit tendencies. In this case, item W7Q16 was removed before fitting the model.
# Optional: Load the mod2 data as a data frame directly from the package file
load(system.file("extdata/mod2.Rdata", package = "D3mirt"))y <- data.frame(x[, -16])# Remove misfitting item W7Q16
# Call D3mirt() and using the orthogonal model
mod2 <- D3mirt(y, modid = list(c(1:10), # x-axis
c(15:19), # y-axis
c(11:14))) # z-axissummary(mod2)
#>
#> D3mirt: 19 items and 5 levels of difficulty
#>
#> Orthogonal model
#> Item vector 1: W7Q1, W7Q2, W7Q3, W7Q4, W7Q5, W7Q6, W7Q7, W7Q8, W7Q9, W7Q10
#> Item vector 2: W7Q15, W7Q17, W7Q18, W7Q19, W7Q20
#> Item vector 3: W7Q11, W7Q12, W7Q13, W7Q14
#>
#> a1 a2 a3 d1 d2 d3 d4 d5
#> W7Q1 2.0183 0.0000 0.0000 8.0211 7.0101 5.9451 3.1762 -0.4884
#> W7Q2 2.5545 0.0000 0.0000 9.0536 6.4408 4.3879 1.6178 -2.3976
#> W7Q3 2.7483 0.0000 0.0000 10.3241 7.4782 5.6007 2.6793 -1.1828
#> W7Q4 1.9121 0.0000 0.0000 7.3485 6.0230 4.9697 2.4782 -1.1279
#> W7Q5 2.1971 0.0000 0.0000 8.2981 6.5636 4.8289 1.7951 -1.8163
#> W7Q6 2.0070 0.0000 0.0000 8.0685 6.3532 4.9445 2.3214 -1.0367
#> W7Q7 1.5979 0.0000 0.0000 5.9310 4.8301 3.6411 1.5984 -1.3322
#> W7Q8 1.7491 0.0000 0.0000 6.8381 5.1224 3.7094 1.4480 -1.8096
#> W7Q9 1.7410 0.0000 0.0000 7.5706 4.9930 3.3754 0.9282 -2.2171
#> W7Q10 1.7488 0.0000 0.0000 8.3054 5.6856 4.2405 1.9417 -0.6614
#> W7Q11 0.0000 0.0000 1.5285 5.9525 4.4684 3.3585 1.1019 -1.7346
#> W7Q12 0.0000 0.0000 1.1511 4.1002 2.8570 2.3265 1.1659 -0.4295
#> W7Q13 0.0000 0.0000 1.8866 5.7126 4.2787 3.3450 1.8306 -0.5882
#> W7Q14 0.0000 0.0000 1.3482 5.4784 4.0172 3.1011 0.8363 -1.6128
#> W7Q15 0.0000 0.8870 0.0000 4.3049 3.0375 2.3797 0.9084 -0.9651
#> W7Q17 0.0000 1.5464 0.0000 6.6917 5.6269 4.7725 2.6404 -0.0421
#> W7Q18 0.0000 1.3316 0.0000 3.6937 2.0504 1.3881 0.1841 -1.9387
#> W7Q19 0.0000 1.6471 0.0000 4.5704 2.6646 1.6572 0.3352 -2.0165
#> W7Q20 0.0000 1.9471 0.0000 4.5578 2.3173 1.2468 -0.3427 -2.8570
#>
#> MDISC MDIFF1 MDIFF2 MDIFF3 MDIFF4 MDIFF5
#> W7Q1 2.0183 -3.9742 -3.4733 -2.9456 -1.5737 0.2420
#> W7Q2 2.5545 -3.5442 -2.5214 -1.7178 -0.6333 0.9386
#> W7Q3 2.7483 -3.7565 -2.7210 -2.0379 -0.9749 0.4304
#> W7Q4 1.9121 -3.8432 -3.1500 -2.5992 -1.2961 0.5899
#> W7Q5 2.1971 -3.7768 -2.9873 -2.1978 -0.8170 0.8267
#> W7Q6 2.0070 -4.0202 -3.1655 -2.4636 -1.1566 0.5165
#> W7Q7 1.5979 -3.7118 -3.0228 -2.2787 -1.0003 0.8337
#> W7Q8 1.7491 -3.9096 -2.9287 -2.1208 -0.8279 1.0346
#> W7Q9 1.7410 -4.3485 -2.8680 -1.9388 -0.5332 1.2735
#> W7Q10 1.7488 -4.7492 -3.2511 -2.4248 -1.1103 0.3782
#> W7Q11 1.5285 -3.8944 -2.9234 -2.1973 -0.7209 1.1348
#> W7Q12 1.1511 -3.5620 -2.4820 -2.0211 -1.0129 0.3731
#> W7Q13 1.8866 -3.0279 -2.2679 -1.7730 -0.9703 0.3118
#> W7Q14 1.3482 -4.0635 -2.9796 -2.3001 -0.6203 1.1962
#> W7Q15 0.8870 -4.8531 -3.4243 -2.6827 -1.0241 1.0880
#> W7Q17 1.5464 -4.3273 -3.6387 -3.0862 -1.7074 0.0272
#> W7Q18 1.3316 -2.7738 -1.5398 -1.0424 -0.1383 1.4559
#> W7Q19 1.6471 -2.7749 -1.6178 -1.0062 -0.2035 1.2243
#> W7Q20 1.9471 -2.3408 -1.1902 -0.6403 0.1760 1.4673
#>
#> D.Cos X D.Cos Y D.Cos Z Theta Phi
#> W7Q1 1 0 0 0 90
#> W7Q2 1 0 0 0 90
#> W7Q3 1 0 0 0 90
#> W7Q4 1 0 0 0 90
#> W7Q5 1 0 0 0 90
#> W7Q6 1 0 0 0 90
#> W7Q7 1 0 0 0 90
#> W7Q8 1 0 0 0 90
#> W7Q9 1 0 0 0 90
#> W7Q10 1 0 0 0 90
#> W7Q11 0 0 1 90 90
#> W7Q12 0 0 1 90 90
#> W7Q13 0 0 1 90 90
#> W7Q14 0 0 1 90 90
#> W7Q15 0 1 0 NaN 0
#> W7Q17 0 1 0 NaN 0
#> W7Q18 0 1 0 NaN 0
#> W7Q19 0 1 0 NaN 0
#> W7Q20 0 1 0 NaN 0In the output, the simple loading structure can be seen. Aldo note
that \(\theta\) reports
NaN when both \(\theta\)
and \(\phi\) is zero. This is because
when converting to spherical coordinates, items oriented parallel to the
\(y\)-axis will have \(\cos (0)\) in the denominator of the \(\arctan\) function, resulting in undefined
values.
The plot() method for objects of class
D3mirt is built on the rgl package (Adler
& Murdoch, 2023) for visualization with OpenGL. Graphing in default
mode by calling plot() will return an RGL device appearing
in an external window as a three-dimensional interactive object
containing vector arrows with the latent dimensions running along the
orthogonal axes that can be rotated. In this illustration, however, all
RGL devices are plotted inline as still shots displayed from two angles,
\(15^{\circ}\) (clockwise; default plot
angle) and \(90^{\circ}\). To change
the plot output to \(90^{\circ}\), use
the view argument in the plot() function and
change the first indicator from \(15\)
to \(90\).
# Plot RGL device with constructs visible and named
plot(mod3, constructs = TRUE,
construct.lab = c("Compassion", "Fairness", "Conformity"))
Figure 1: Three-dimensional vector plot for all items and the three constructs Compassion, Fairness, and Conformity (solid black arrows) plotted with the model rotated \(15^{\circ}\) clockwise.
Figure 2: Three-dimensional vector plot for all items and the three constructs Compassion, Fairness, and Conformity (solid black arrows) plotted with the model rotated \(90^{\circ}\) clockwise.
An example of how the output can be described is as follows.
As can be seen in Figures 1 and 2, the pattern in the data indicates the presence of foremost two main nested latent constructs indicated by the items, one aligned with the \(x\)-axis and one approaching the \(y\)-axis. We might also suspect the presence of a third construct located close to the \(xy\)-plane, between the \(x\) and \(z\) axes. Studying the content of the items, the labels Compassion, Fairness, and Conformity were introduced. The angles of the constructs inform us that Compassion (\(\theta = 2.092^{\circ}\), \(\phi = 86.061^{\circ}\)) and Conformity (\(\theta = -2.514 ^{\circ}\), \(\phi = 28.193^{\circ}\)) have some within-multidimensional tendencies. However, they are both more or less orthogonal to the \(z\)-axis. Next, we find Fairness (\(\theta = 49.101^{\circ}\), \(\phi = 78.313^{\circ}\)) with clear within-multidimensional tendencies with respect to the \(x\)-axis. Thus, the output indicates that Compassion and Conformity could be independent constructs but Fairness seems not to be.
The orthogonal model is displayed below rotated rotated \(15^{\circ}\) clockwise. In the graphical output the perpendicular orientation of the items forms a cross pattern in which each axis holds a unidimensional model indicated by the items parallel to it. The orthogonal model is foremost useful for studying the items under the assumption that the items are unidimensional and that the unidimensions are uncorrelated. Note that the orthogonal model will change the location of respondents in the latent space compared to the compensatory model. This can have an effect when studying profiles (see examples below), such that profile tendencies can emerge or disappear depending on the choice of model.
# Plot the orthogonal model
plot(mod2)
Figure 3: The orthogonal model plotted with the model rotated \(15^{\circ}\) clockwise.
itemsA subset of items can be plotted for a more thorough investigation
using the items argument. In the example below, all
constructs are plotted together with the items used for the conformity
construct. In the function call, the numerical indicators in the
items argument follow the item order in the original data
frame (see ?anes0809offwaves).
# The Conformity items from the model plotted with construct vector arrows
plot(mod3, constructs = TRUE,
items = c(15,17,18,19,20),
construct.lab = c("Compassion", "Fairness", "Conformity"))
Figure 4: The items from the Conformity construct plotted with the model rotated \(15^{\circ}\) clockwise.
Figure 5: The items from the Conformity construct plotted the model rotated \(90^{\circ}\) clockwise.
The plot() function also allows plotting a single item
by entering a single number indicating what item that should be
displayed. As was mentioned above, the W7Q16 was not included in any of
the constructs because the item showed signs of measurement problems.
For example, the short vector arrows indicate high amounts of model
violations and the location of the item in the model also indicates that
the item is within-multidimensional and does not seem to belong to any
construct explicitly. Typing \(16\) in
the items argument allows for a closer look.
# Item W7Q16 has location 16 in the data set (gender and age excluded)
# The item is plotted together with construct to aid the visual interpretation
plot(mod3, constructs = TRUE,
items = 16,
construct.lab = c("Compassion", "Fairness", "Conformity"))
Figure 6: The item W7Q16 plotted with the three constructs and with the model rotated \(15^{\circ}\) clockwise.
Figure 7: The item W7Q16 plotted with the three constructs and with the model rotated \(90^{\circ}\) clockwise.
An example of how the output for analysis of the single item above is as follows.
The Figures 3 and 4 shows that item W7Q16 is located at \(\theta = 16.085^{\circ}\), \(\phi = 57.476^{\circ}\), indicating that the item is within-multidimensional with respect to the \(x\) and \(y\)-axis; but much less so with respect to the \(z\)-axis. In addition, the directional discrimination further underscores that the item does not seem to measure any particular construct (\(DDISC_1 = .657\), \(DDISC_2 = .617\), \(DDISC_3 = .656\)). The global discrimination (\(MDISC = .770\), \(MDIFF_{range} = [-4.838, 2.349]\)) is also the lowest of all discrimination scores in the model. This, combined, implies that the item in question does not seem to fit the three-dimensional DMIRT model used in this analysis and should, therefore, be removed or adapted. We can also note that item W7Q15, \(MDISC = .923\), \(MDIFF_{range} = [-4.680, 1.051]\)) has the second lowest global discrimination score. Compared to W7Q16, however, item W7Q15 does seem to belong to the Conformity construct, observable when comparing angle orientation (\(\theta = -19.432^{\circ}, \phi = 31.515^{\circ}\)) and direction discrimination (\(DDISC_1 = .502\), \(DDISC_2 = .332\), \(DDISC_3 = .912\)). In other words, even if item W7Q15 shows signs of statistical violations in the model, the item still informs us of the content of the Conformity construct.
diff.levelThe user has the option of plotting on one level of difficulty at a
time with the diff.level argument studying the entire
scale, a subset of items, or on one item at a time. Note that
difficulty refers to the number of item response functions in
the items, i.e., the total number of response options minus one. In this
case, \(6\) response options were used
which means that the model has \(5\)
levels of difficulty.
# Plot RGL device on item difficulty level 5
plot(mod3, diff.level = 5)
Figure 8: All items plotted on difficulty level 5 and with the model rotated \(15^{\circ}\) clockwise.
Figure 9: All items plotted on difficulty level 5 and with the model rotated \(90^{\circ}\) clockwise.
scaleThe D3mirt() function returns item vector coordinates
estimated with and without the \(MDISC\) as a scalar for the arrow length.
When the \(MDISC\) is not used for the
arrow length, all item vector arrows are scaled to one unit length. This
allows the user to graph the item vector arrows with plot()
set to a uniform length. This can help reduce visual clutter in the
graphical output. To view the item vector arrows without the \(MDISC\), set scale = TRUE.
# Plot RGL device with items in uniform length and constructs visible and named
plot(mod3, scale = TRUE,
constructs = TRUE,
construct.lab = c("Compassion", "Fairness", "Conformity"))
Figure 10: All items scaled to uniform length and plotted with the model rotated \(15^{\circ}\) clockwise.
Figure 11: All items scaled to uniform length and plotted with the model rotated \(90^{\circ}\) clockwise.
D3mirt Profile
AnalysisThe plot() function can also display respondents in the
three-dimensional model represented as spheres whose coordinates are
derived from the respondent’s trait scores. This allows for a profile
analysis in which respondents can be separated and plotted as subsets
conditioned on single or multiple criteria. The resulting output shows
where the respondents are located in the model and, accordingly, what
model profile best describes them. To do this, the user can either plot
all respondents by setting ind.scoresto TRUE,
or plot a subsection of respondents by creating a separate data frame
and use it in the profiles argument when calling
plot().
In the example below, the first option is illustrated also using the
levels argument. Regarding the latter, the
plot() function uses as.factor() to count the
number of factor levels in the data imputed in the levels
argument. This information is then used to assign colors for the spheres
representing respondents when plotting. This means that raw data can be
used as is, but the number of colors in the color vectors argument
(sphere.col) may need to be adapted. In the example below,
the criteria variable for gender only hold two factor levels and,
therefore, only two colors in the color vector are needed. By separating
respondents using color coding, it is sometimes possible to display
group-level effects.
Generally, it can be helpful to hide vector arrows with
hide = TRUE when plotting respondent profiles to avoid
visual cluttering. The example below displays all respondents using the
gender variable included in the built-in data set. The gender variable
is also color-coded so that males are displayed as blue spheres and
females as red spheres.
# Load the data set to be used in levels
# Use as.matrix to remove any attributes
data("anes0809offwaves")
x <- as.matrix(anes0809offwaves)Call plot() and use the gender data, contained in column
two in data frame \(x\), in the
levels argument. In the function call below, the axes in
the model are named using the x.lab, y.lab,
and z.lab arguments following the direction of the
constructs. Note that the model axes represent unidimensional singular
structures or traits.
# Plot profiles with item vector arrows hidden with hide = TRUE
# Score levels: 1 = Blue ("male") and 2 = Red ("female")
# Plot profiles with item vector arrows hidden
# Score levels: 1 = Blue ("male") and 2 = Red ("female")
plot(mod3, hide = TRUE, ind.scores = TRUE,
levels = x[, 2],
sphere.col = c("blue", "red"),
x.lab = "Compassion",
y.lab="Conformity",
z.lab="Fairness")
Figure 12: Gender profile for the anes0809offwaves data
set plotted with the model rotated \(15^{\circ}\) clockwise.
Figure 13: Gender profile for the anes0809offwaves data
set plotted with the model rotated \(90^{\circ}\) clockwise.
An example of how the output can be described is as follows.
In the figure, a simple gender profile can be observed, showing that more women tend to have higher levels of compassion. When rotating the model \(90^{\circ}\) clockwise, there seems to be no obvious gender difference related to Conformity or Fairness.
It is also possible to plot a confidence interval in the shape of an
ellipse surrounding the spheres in the latent space. In the example
below, the younger individuals (\(\leq30\)) are selected and plotted with a
\(95\%\) CI. To subset
respondents, create a new data frame by combining respondents’ factor
scores from mod3$fscores with age data from column one in
data frame \(x\). Then assign
respondent data conditioned on age to a new data frame using the
subset() function.
# Column bind trait scores in mod3 with the age variable W3Xage from data frame x
z <- data.frame(cbind(mod3$fscores, x[, 1]))
# Subset data frame z conditioned on age <= 30
z1 <- subset(z, z[, 4] <= 30)When a criterion variable has a wide data range, such as an age
variable, rep() can be used to set the appropriate size of
the color vector for sphere.col by repeating color names
with rep(). When plotting, the plot() function
will pick colors from the sphere.col argument following the
factor order in the levels argument. To do this, the first step is to
count the number of factors in the criterion variable. This can be done
with nlevels(), as illustrated below.
# Check number of factor levels with nlevels() and as.factor()
nlevels(as.factor(z1[, 4]))
# Use rep() to create a color vector to color groups based on the nlevels() output
# z1 has 14 factor levels
colvec <- c(rep("red", 14))To plot the CI, the ci argument is set to
TRUE. The color of the sphere was also changed from default
grey80 to orange in the example below. Note
that the CI limit can be adjusted with the
ci.level argument.
# Call plot() with profile data on age with item vector arrows hidden
plot(mod3, hide = TRUE,
profiles = z1,
levels = z1[, 4],
sphere.col = colvec,
x.lab = "Compassion",
y.lab="Conformity",
z.lab="Fairness",
ci = TRUE,
ci.level = 0.95,
ellipse.col = "orange")
Figure 14: Adults less than or equal to age 30 from the
anes0809offwaves data set plotted surrounded by a \(95\%\,CI\) and with the model rotated \(15^{\circ}\) clockwise.
Figure 15: Adults less than or equal to age 30 from the
anes0809offwaves data set plotted surrounded by a \(95\%\,CI\) and with the model rotated \(90^{\circ}\) clockwise.
An example of how the output can be described is as follows.
In Figures 7 and 8 we can see a tendency for an age profile in which younger individuals could be described as less oriented towards Conformity. We can also observe a tendency for what could be an interaction effect in which higher levels of Conformity seem to be associated with lower levels of Fairness.
Some options for exporting the RGL device are shown below. In
addition, it is also possible to export graphical devices in R Markdown
documents with rgl::hook_webgl() together with graphical
options for knitr, as was done when creating the package vignette.
# Export an open RGL device to the console that can be saved as an html or image file
plot(mod3, constructs = TRUE)
s <- rgl::scene3d()
rgl::rglwidget(s,
width = 1040,
height = 1040)
# Export a snap shoot of an open RGL device directly to file
plot(mod3, constructs = TRUE)
rgl::rgl.snapshot('RGLdevice.png',
fmt = 'png')If you encounter a bug, please file an issue with a minimal reproducible example on GitHub (https://github.com/ForsbergPyschometrics/D3mirt). For questions and suggestions on how to improve the code, please contact me on GitHub or via email (forsbergpsychometrics@gmail.com).
Adler, D., & Murdoch, D. (2023). Rgl: 3d Visualization Using OpenGL [Computer software]. https://dmurdoch.github.io/rgl/index.html
Chalmers, R., P. (2012). mirt: A Multidimensional Item Response Theory Package for the R Environment. Journal of Statistical Software, 48(6), 1-29. https://doi.org/10.18637/jss.v048.i06
DeBell, M., Krosnick, J. A., & Lupia, A. (2010). Methodology Report and User’s Guide for the 2008-2009 ANES Panel Study. Palo Alto, CA, and Ann Arbor, MI: Stanford University and the University of Michigan.
McKinley, R. L., & Reckase, M. D. (1983). An Extension of the Two-parameter Logistic Model to the multidimensional latent space, Report ONR83-2. Iowa City, IA, American College Testing Program.
Muraki, E., & Carlson, J. E. (1995). Full-Information Factor Analysis for Polytomous Item Responses. Applied Psychological Measurement, 19(1), 73-90. https://doi.org/10.1177/014662169501900109
Reckase, M. D. (2009). Multidimensional Item Response Theory. Springer. https://doi.org/10.1007/978-0-387-89976-3
Reckase, M. D. (1985). The Difficulty of Test Items That Measure More Than One Ability. Applied Psychological Measurement, 9(4),401-412. https://doi.org/10.1177/014662168500900409
Reckase, M. D., & McKinley, R. L. (1991). The Discriminating Power of Items That Measure More Than One Dimension. Applied Psychological Measurement, 15(4), 361-373. https://doi.org/10.1177/014662169101500407