st: how to create an average of previous observations

[Daniel Simon <dhs29@cornell.edu>]

Hi  - I have data on a variable, X, for a set of individuals. Each 
observation also has the date that X was recorded. I want to create a new 
variable, X_AVG, which is the mean value of X for all observations with a 
date that precedes the date for the current observation. X_AVG would be a 
moving average, that would incorporate all observations with earlier dates.

Example:

Obs	Date		X	X_AVG
1	1/1/2004	4	.
2	1/2/2004	3	4
3	1/4/2004	5	3.5
4	1/5/2004	2	4
5	1/8/2004	8	3.5
6	1/10/2004	5	4.4
7	1/10/2004	8	4.4
8	1/11/2004	3	5

I want to know how to create X_AVG. And, in particular, is there a way to 
deal with cases like observation 7, which has the same date as observation 
6. In that case, I would not want to use the value of X in observation 6 to 
create X_AVG for observation 7.

I hope that this is clear enough. Thanks. Daniel

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