---
title: "Biostatistics III in R"
author:
- Code by Johan Zetterqvist, Andreas Karlsson and Mark Clements
output:
  prettydoc::html_pretty:
    theme: cayman
    highlight: github
---

# Exercise 6. Diet data: tabulating incidence rates and modelling with Poisson regression #

```{r setup, cache=FALSE, message=FALSE, echo=FALSE}
library('knitr')
read_chunk('../q6.R')
opts_chunk$set(cache=FALSE)
```

You may have to install the required packages the first time you use them. You can install a package by `install.packages("package_of_interest")` for each package you require.

```{r loadDependencies, message=FALSE}
```
Load the diet data using time-on-study as the timescale. We look at the first six rows of the data using `head` and look at a summary for the variables using `summary`:

```{r loadPreprocess, results='asis'}
```

## (a) ##

```{r 6a_ir,  message=FALSE, results='asis'}
```

```{r 6a_ir_b,  message=FALSE}
```

We see that individuals with a high energy intake have a lower
        CHD incidence rate. The estimated crude incidence rate ratio
        is 0.52 (95% CI: 0.27, 0.97).

## (b) ##

The code is:

```{r 6b_ir}
```

The point estimate for the IRR calculated by the Poisson regression is
the same as the IRR calculated in 6(a). The regression model can be defined by:

\begin{align*}
E(\text{chd}) &= \frac{y}{1000}\exp\left(\beta_0 + \beta_1I(\text{hieng}="high") \right) \\
&= \exp\left(\beta_0 + \beta_1I(\text{hieng}="high") + \log(y/1000) \right)
\end{align*}

where $E(\text{chd})$ is the expected count for CHD, $\beta_0$ is the
intercept parameter for the log rate and $\beta_1$ is the parameter
for the log rate ratio for high energy diets. We have also used
$I(\text{condition})$ as an indicator function, which will take a
value of 1 when the condition is true and 0 otherwise.

A theoretical observation: if we consider the data as being
cross-classified solely by `hieng` then the Poisson regression model
with one parameter is a _saturated model_ (that is, the number of
observations is equal to the number of parameters) so the IRR
estimated from the model will be identical to the ‘observed’ IRR. That
is, the model is a perfect fit.

## (c) ##

```{r 6c_energyDist,  message=FALSE}
```

The histogram and density curve gives us an idea of the distribution of energy intake. The normal curve suggests that a normal approximation may be
acceptable. We can also tabulate moments and percentiles of the
distribution.

## (d) ##

```{r 6d_engCat}
```

This could also be done using `dplyr::mutate`.

## (e) ##


```{r 6e_irEng}
```

We see that the CHD incidence rate decreases as the level of total
energy intake increases. These calculations are usually better done
using a regression model as they are less fiddly and can be adjusted
for other covariates.

## (f) ##


```{r 6f_irEng, results='asis'}
```

## (g) ##

```{r 6g_irEng}
```

## (h) ##

```{r 6h,  message=FALSE}
```

Level 1 of the categorized total energy is the reference category. The
estimated rate ratio comparing level 2 to level 1 is 0.6452 and the
estimated rate ratio comparing level 3 to level 1 is 0.2886.

The regression equation can be represented by:

\begin{align*}
E(\text{chd}) &= \frac{y}{1000}\exp\left(\beta_0 + \beta_2 X_2 + \beta_3 X_3 \right) \\
&= \exp\left(\beta_0 + \beta_2 X_2 + \beta_3 X_3 + \log(y/1000) \right)
\end{align*}
where $\beta_2$ and $\beta_3$ are the log rate ratios for $X_2=\text{X2}$ and $X_3=\text{X3}$, respectively.

## (i) ##


```{r 6i,  message=FALSE}
```

Now use level 2 as the reference (by omitting X2 but including
        X1 and X3). The estimated rate ratio comparing level 1 to level
        2 is 1.5498 and the estimated rate ratio comparing level 3 to
        level 2 is 0.4473.

## (j) ##

```{r 6j,  message=FALSE}
```

The estimates are identical (as we would hope) when we have
        R create indicator variables for us.

## (k) ##

Somehow (there are many different alternatives) you’ll need to
        calculate the total number of events and the total person-time
        at risk and then calculate the incidence rate
        as events/person-time. As examples,

```{r 6k}
```

The estimated incidence rate is 0.00999 events per person-year.
