Exhibit 30 Rolling 6m return CFAI p.84

On page 84, CFAI vol.5, in exhibit 30 there is an example of how to calcualte a rolling 6m returns mean.

For the mean, in stead of 6 periods, 7 periods are taken for the average calculation.

What is the difference between simply calculating the rolling 6m return (i.e. n=6 periods), vs. a roll return mean i.e. why add the additional period.

I hope anyone can help me with this.