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arii
Obsidian | Level 7

Hi, 

 

I need to test the difference in mean between two time periods in my data set. However, I need to carry out this test multiple times.

 

Precisely, for each 'date' in my data; I need to test the difference between the mean of 'X1' for '30 days before' and '30 days after'.

 

The code below works perfectly, but I need to manually specify each date (which is obviously too cumbersome). I would sincerely appreciate some suggestions!

 

 

Present Code:

/*date of interest: 16May2001*/

 

data xx;
set x;

 

Startdate1 = '16May2001'd - 30;
Enddate1 = '16May2001'd;

 

Startdate2 = '17May2001'd;
Enddate2 = '17May2001'd + 30;

 

If Startdate1 < date < Enddate1 then flag=0;
If Startdate2 < date < Enddate2 then flag=1;

 

run;

 

 

proc ttest data=xx;

 

class flag;

var X1;
run;

 

 

 

Sample Data:

 

(Date Format: mmddyear)

 

DateX1
1/2/200122
5/16/200134
5/26/200165
5/27/200111
6/8/200156
10/11/200131
12/12/200191
1/13/200449
7/18/200556
9/19/200621
12/2/200767
12/21/200734
12/29/200767
1/23/200890
7/14/200878
9/25/200845
6/28/200945
1/29/201054

 

 

Thank you in anticipation!

7 REPLIES 7
PGStats
Opal | Level 21

Here is one approach using a self join:

 

data have;
input Date :mmddyy. X1; 
format date yymmdd10.;
datalines;
1/2/2001 22 
5/16/2001 34 
5/26/2001 65 
5/27/2001 11 
6/8/2001 56 
10/11/2001 31 
12/12/2001 91 
1/13/2004 49 
7/18/2005 56 
9/19/2006 21 
12/2/2007 67 
12/21/2007 34 
12/29/2007 67 
1/23/2008 90 
7/14/2008 78 
9/25/2008 45 
6/28/2009 45 
1/29/2010 54 
;

proc sql;
create table test as
select
    a.date as middate,
    b.date > a.date as period,
    b.date,
    b.x1
from 
    have as a inner join
    have as b 
      on b.date between intnx('day',a.date,-30) and intnx('day',a.date,30) and
      a.date ne b.date
group by middate
having count(distinct period) = 2
order by middate, date;
quit;

ods select none;

proc ttest data=test plots=none;
ods output ttests=tt;
by middate;
class period;
var x1;
run;

ods select all;

proc print data=tt; run;
PG
arii
Obsidian | Level 7

Thank you for your response, PG Stats.

 

Is there a way to do this without the 'datalines' in the code? My actual data has close to 500 observations and the 'datalines approach' would be kinda clumsy for me.

 

 

PGStats
Opal | Level 21

Of course, there are many ways. The first step in the code above is just there for testing. Create your have dataset with a datastep using infile or a SQL query or any other means. As long as it contains the date and x1 variables.

PG
arii
Obsidian | Level 7

Hi PGStats,

 

Unfortunately, I am still struggling with the codes you supplied (please pardon my basic SAS knowledge).

 

I am using the data-set approach because I have already created a data-set from my main data file. I have over a thousand observations, I don't think the 'datalines approach' would work well for me.

 

However, I still have lots of errors in the log file. I am not too sure what is wrong.

 

 

See the modified code I used below:

 

data xxx;
set xx;

 

input Date :mmddyy. X1;
format date yymmdd10.;

 

proc sql;
create table test as
select
a.date as middate,
b.date > a.date as period,
b.date,
b.x1
from
have as a inner join
have as b
on b.date between intnx('day',a.date,-30) and intnx('day',a.date,30) and
a.date ne b.date
group by middate
having count(distinct period) = 2
order by middate, date;
quit;

ods select none;

 

proc ttest data=test plots=none;
ods output ttests=tt;
by middate;
class period;
var x1;
run;

ods select all;

 

proc print data=tt; run;

 

 

See some errors from log file:

 

ERROR: No DATALINES or INFILE statement.
NOTE: The SAS System stopped processing this step because of errors.

 

 

 

ERROR: Variable MIDDATE not found.

 

 

 

ERROR: File WORK.TT.DATA does not exist

 

PGStats
Opal | Level 21

The first thing you must do is to bring your data into a SAS dataset. That is often the first difficulty that new users encounter. Where is your main data file?

PG
arii
Obsidian | Level 7

I have attached the main data file herein.

 

 

arii
Obsidian | Level 7

I doubt I had issues with the data import. My main challenge was with conducting the 'proc t test' multiple times - using the dates in my data file as a reference.

 

I have attached my full codes herein.

 

Thank you for your help, PGStats.

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