st: simple calculations across rows

["B. Timothy Walsh" <btw1@columbia.edu>]

Hello!
I am a beginner-to-intermediate level STATA user, and would like to find a 
solution to what I hope is a relatively straightforward problem. I have 
looked a bit at the archives, and have been led in the direction of mata, 
but am wondering if another (less daunting?) method might be available.

I am importing data from a device and would like to use STATA to do some 
data reduction. The device monitors types of physical activity over 2 days, 
and the data are provided in rows, each of which contains a summary of data 
over pre-defined 4 hour blocks, like 8 AM to 12N. So, each subject has 6 
rows of data/day and a total of 12 rows for 2 days.

What I wish to do is to be able to summarize--like simple averages, 
etc.--across some but not all time periods, and, maybe, days. For example, 
for each subject, calculate the total activity over the 8AM to 4PM blocks 
on day 1 and compare that to the same measure on day 2. Etc.

Here is a somewhat simplified outline of how one subject's data might look 
for the first 3 periods of day 1:

ID	DAY		PERIOD	MEASURE
i	1		1		Xi11
i	1		2		Xi12
i	1		3		Xi13

I have only a modest dataset, so I do not need particularly efficient code. 
Better code that is easy to interpret and be modified by a novice such as 
myself.

Thank you for any guidance!
Tim Walsh

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