Two things I have learnt today:
1. The effect on productivity of days taken off work sick is measured using the Bradford Factor, which applies the principle that taking one day off many times is more detrimental than a single extended absence. I currently have a Bradford Factor of 0.0.
2. How to use Excel's SUMIF formula. Very satisfying. I knew about COUNTIF already but rather than the number of non-blank cells in one column, I wanted to use that information to add up the related values in a different column.