I am not following the rationale in the answer for the average accident date in 06' - would someone be able to explain? Thanks Ed
There are 1,000 policies on risk during Sep, 2,500 in Oct, 4,500 in Nov and 7,000 in Dec. You can think of these as like exposure units. During each month, the average accident date will be mid-month (so mid-Sep, mid-Oct, etc). Taking a weighted average of these dates with the corresponding exposure units for each month, gives the overall average accident date 2/3rds the way through November, ie (8.5*1000+9.5*2500+10.5*4500+11.5*7000)/15000 = 10.6667 (ps I have changed the title of the thread so people can search for the question )