New Contributor
Posts: 2

# Duration analysis with longitudinal data

Hello,

I am a new user of SAS so that my question may be very easy but unfortunately I do not know the aswer. I will appreciate for any of your suggestions.

Thank you very much for your time and Merry Christmas!

The following is the description of the dataset and the results that I would like to find:

Data description: longitudinal data, edu is a dummy which 1 represents to have education in the corresponding tax year; 0 not.

I would like to get (1) the duration of the waiting period for a person to go to education, and the number of years s/he participated in education. (2) Also, the mean of the waiting time and the number of years in edu for all people.

For (1), I would like to have a variable to show the first person wait for 1 year to start education, and stay in school for 2 years; the 2nd person waits 0 year and stay in school for 3 years, 4th wait for 2 years and in school for 2 years. For person 3, he has multiple entry of education but I would like to count only the first time and treat the 2nd or any furhter edu as no edu in the period.

For (2), I would like to think it is good if the mean of waiting time and years of edu can be calculated directly.

Sample data&colon;

id            tax_year              edu

1              1                              0

1              2                              1

1              3                              1

1              4                              0

1              5                              0

2              1                              1

2              2                              1

2              3                              1

2              4                              0

2              5                              0

3              1                              0

3              2                              1

3              3                              0

3              4                              1

3              5                              0

4              1                              0

4              2                              0

4              3                              1

4              4                              1

5              5                              0

Super User
Posts: 6,648

## Re: Duration analysis with longitudinal data

Calculating means is easy.  Preparing the numbers is a bit trickier.  Assuming your data is sorted as indicated, BY ID TAX_YEAR:

data want;

set have;

by id;

if first.id then do;

waiting_increment=1;

education_increment=1;

waiting_years=0;

education_years=0;

end;

retain waiting_increment education_increment;

if edu=0 then do;

if education_years > 0 then education_increment=0;

waiting_years + waiting_increment;

end;

else do;

waiting_increment=0;

education_years + education_increment;

end;

if last.id;

run;

Then run PROC MEANS to get the means:

proc means data=want;

var waiting_years education_years;

run;

It's untested ... should be fine by write back if there are any difficulties.

New Contributor
Posts: 2

## Re: Duration analysis with longitudinal data

Thanks. Astounding.

I tested the code and it works. I am wondering whether I can ask you the logic behind the code. Please forgive me if my question is too basic.

I did a few tests and try to find the logic when SAS runs this code. I think SAS handles it as kind of loop. The reason that I think so is that: before running the "else do....end" part, the code generates the total of all non-edu period, or the counts for all edu = 0. However, when the else do...end added, the periods after an edu = 1 have been deducted from the previous totals. This generates the actual waiting time. The reason I think it is a loop is from here-- the Else do changed the condition of calculating total and recalculated it. The conclusion also comes from the education_years, since when SAS run the “If edu_years >0 then …” , all edu_years are equal to zeros.

I am wondering whether you can point out my understanding is correct or not.

Thank you very much for your time.

Contributor
Posts: 61

## Re: Duration analysis with longitudinal data

Try this: edu2 and edu3 datasets will have appropriate variables you need. You can then use any of the procs univariate/means to count means.

``````data edu2(keep=id edu1_yr) edu3(keep=id tot_edu);
set edu;
by id tax_year;
where edu=1;
retain tot_edu;
new_yr=lag(tax_year);
if first.id then new_yr=tax_year;
if edu=1 and tax_year-new_yr>1 then new_edu=0;
else new_edu=edu;
if first.id then do;
edu1_yr=tax_year-1;
output edu2;
end;
if first.id then tot_edu=0;
tot_edu=tot_edu+new_edu;
if last.id then output edu3;
run;``````

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