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st: Rolling calendar estimates

From   "Yvonne Capstick" <>
Subject   st: Rolling calendar estimates
Date   Tue, 22 Mar 2005 14:00:43 -0500


I have a dataset of companies' stock returns on a daily basis. It is easy to calculate the standard deviation of the returns for each company per year, using:

egen sigma = sd(ret), by (firmyear)

However, if I wish to do a rolling estimate giving the standard deviation of the last actual rather than calendar year (e.g. if today is 22 March I want the standard deviation from 22 Mar 04 to 21 Mar 05, not from 1 Jan 04 to 31 Dec 05), is there an easy way to do that? If I know there are 252 trading days per year, can I just ask it to calculate the std dev of the last 252 options? I'd like to do something like

egen sigma = sd(ret[_n-252]:ret[_n-1]) if firm[_n-252] == firm[_n]

but it doesn't seem to allow anything like that.

Many thanks,

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