Bradford Factors are used by some to measure the significance of absenteeism and are computed for each individual as S squared * D where S is the number of spells (continuous periods of absence) and D is the sum of the days. The calculation is often made over a rolling 52 weeks.

Commercial HR software often has the facility to calculate these factors but a Google search indicates quite a lot of interest without any free solutions.

Using units of half a day and including any non-working days in each spell, how does one calculate the factors using Excel 2007?