Chapter 4 feedback

[tombisho]

This chapter details the analytical prerequisites so might be best as a subsection elsewhere?
At the moment there isn't any context about the dataset or the example that is being followed. This could either belong here or in a previous chapter. For example, say that there are 3 data sets, held on the same server but can be considered separate, there are X people in each data set, the variables are X,Y,Z and we will look at how A,B,C affect survival time, meta analysing the results?

(as a side point a feature of dsSurvival in the future could be to validate all these things e.g. check event variable is numeric and provide useful feedback - if it doesn't do this already?)