Perhaps you're getting confused by the heading for this column. They've simply calculated the number of policies in force during each month. So: 1000 policies in force during September, with an average claim date at time 8.5 (ie mid Sept) 2500 policies in force during October, with an average claim date at time 9.5 (ie mid Oct) 4500 policies in force during November, with an average claim date at time 10.5 (ie mid Nov) 7000 policies in force during December, with an average claim date at time 11.5 (ie mid Dec) Then they take the weighted average of these times, weighting by the number of policies in force: (8.5*1000 + 9.5*2500 + 10.5*4500 + 11.5*7000)/(1000+25001+4500+7000)
