How to take the average of values only if the identifier is the same?

Hello,

I am trying to calculate the average of values that correspond to the same identifier. How could I do that?

I have a forecast for 11 or 12 months for the same firm and then the actual value that was realized for that year.

I would like to compute the average of the forecast values for each unique firm-year (that is the ID) and then subtract the average from the actual (realized) value.

I have uploaded an excel file with the forecasts/actuals and I have also indicated the desired outcome.

Thanks!