Thank you for your kind response. I know the methodlogy of 10% and grouping method. The point that I'd like to know is the difference between description and equation.

I think that the equation for grouping should be as follows to meet the descriptoin.

R= sqrt(sum(Rx^2) + sum(sum(Ri))^2 )
First part Rx : x means the mode that wide spaced exclude close space

Second
Ri : i mean close spaced mode of q th group