Introduction

WHO performs the Rasch Analysis for the Model Disability Survey using the software R. R is an open source, statistical programming software. How to program in R is beyond the scope of this guide. However the codes we will discuss as follows have been written in such a way to require minimal programming knowledge, and this guide tries to make the instructions for how to use them as simple as possible.

To learn about R, please see the references provided at the end of this guide.

To use the package provided by WHO, R must be installed from the Comprehensive R Archive Network (CRAN). We also recommend installing RStudio, a very popular integrated development environment (IDE), for R. One can think of the relationship between R and RStudio like a car: R is the engine of the car, while RStudio is the dashboard and controls. RStudio cannot be run without R, but it makes R easier to use. Both are free to install and use indefinitely.

What is a package and how do I install one?

A “package” is a collection of R functions, data and code with a specific purpose and in a well-defined format. There are thousands of packages for R that have been written by R users. People write packages in order to share codes they have written that make specific tasks easier. WHO has written a package in order to make analysis of the data from the MDS easier. The package written by WHO is called whomds. To install the package, run the following code in the console:

install.packages("whomds")

Using whomds

Once whomds is installed, it can be opened with the following code:

library(whomds)

whomds contains three kinds of functions:

• table_*() functions - produce different fit-for-purpose tables
• fig_*() functions - produce different fit-for-purpose figures
• rasch_*() functions - used when performing Rasch Analysis

In this section we will focus on the rasch_*() functions.

There are two sets of rasch_*() functions in the whomds package: one set for adults, and one set for children. The reason different functions are needed for each group is that the type of Rasch Analysis performed for each group is different. The Rasch Analysis performed for adults is straightforward; the whole adult population can be considered in one group. The Rasch Analysis for children requires a more complicated analysis, because children at different ages are very different, and they can perform vastly different types of activities. First we will describe the analysis of adults, and then describe the analysis of children.