Standard Deviation /Volatility of Sales Growth

[lal mohan kumar]

Dear all
I need to take the standard deviation of every realization of annual sales growth over the past 3 years prior to year t for the firm ' i' as sales_volatility_{it .}
Assume Sales growth =(Salesgrowth_t-Salesgrowth_t-1)/Salesgrowth_t-1
My question is how to generate sales_volatility_it ?

I used the following commands

**id stands for firm id**

gen sg= d.sales/l.sales
label var sg "sales growth"

ssc install tsegen
tsegen sd_sg= rowsd(L(1/3).sg)
label var sd_sg "sales_volatilityi"

Are my commands correct? Any help in this regard will be highly helpful