st: summation (+ absolute value) formula

["west--\@libero\.it" <west--@libero.it>]

I need some help, thank you in advance. 

Given a variable (taken from a set of n variables in a panel), called "Pippo", I need to compute the following formula:

Somma_(i=t-2 to t) of Absolute_value[(Pippo(i) - Pippo(i-1))/Pippo(i-1)]


ATB,

D.







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