Weight values over time

You could do it by:

• Creating an attendance table so there would be 1 row for each rep per month (you could go down to day level if you have the data)
• Each month append a new set of data for reps still with you
• Include a column with their "score" i.e number of months since start date/12 with a max value of 1.

Something like

Month|Rep_ID|Score

1|1|1

1|2|0.2

2|1|1

2|2|0.3

Then just aggregate the scores by months. This approach would also allow reps to have gaps in thier employment i.e. sickness etc.