st: Screening in date variables

["kelly johnson" <econometrics@hotmail.com>]

Hi,

I have a dataset, with thousands of observations) as follows:


obs	Date	ID	        X
1	 7/1/05 	1	0.2
2	 7/2/05	1	0.3
3	 7/3/05	1	0.2
4	7/4/05	1	0.1
5	7/5/05	1	0.5
6	7/6/05	1	0.6
7	 6/16/05	2	0.4
8	 6/17/05	2	0.4
9	 6/18/05	2	0.4
10	6/19/05	2	0.2
11	6/20/05	2	0.1
12	6/21/05	2	0.3
13	6/22/05	2	0.5
14	 4/11/05	3	0.2
15	 4/12/05	3	0.8
16	 4/13/05	3	0.7
17	4/14/05	3	0.8
18	4/15/05	3	0.2
19	4/16/05	3	0.2
20	4/17/05	3	0.3
21	4/18/05	3	0.1
22	4/16/05	3	0.5
...
1000

The ID varible is unique to each patient, and identifies them. The X 
variable is a metric for each day. I want to test all the observations in 
this dataset to see (1) how they performed during caertain time periods 
(i.e. how any patient's X changed between the period 7/2/05-7/4/05, etc). 
First I have to somehow screen for that. If they were in the given date 
range, then I have to calculate the percentage change in X. Also, I want to 
calculate the percentage change in X for each observation (as identified by 
the variable ID).  The trouble I am having is that the trial periods are of 
different lengths. Any help would be appreciated!

Thank you!

Fuaad

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