topic Creating a random Sample under constraint in Statistical Procedures

Creating a random Sample under constraint

fedeava — Fri, 08 May 2020 08:09:36 GMT

Hello everyone,

I need to create a stratified sample starting from a wider population. But in creating the sample I have the constraint that I can choose observations only on a subset of the entire population. However, at the end the stratification variables should reflect the same proportions of the whole population and not of the population subset. I hope I explained.

I have tried to use the proc surveyselect with the where condition but the final proportions don't reflect the whole population.

proc surveyselect data=pop_new_14_17_stima (where=(FLAG_INVIO_BFD=1)) method=sys rate=0.9
seed=1953 out=camp_new rep=1 ;
strata DAT_FINE_PERIO tipo_cli_ps DEFAULT;
run;

Thanks!

Re: Creating a random Sample under constraint

FreelanceReinh — Fri, 08 May 2020 11:07:18 GMT

Hello @fedeava and welcome to the SAS Support Communities!

I think the ALLOC= option of the STRATA statement is suitable for this purpose.

Here's an example:

Let's say we want to draw an 80% sample (without replacement) from SASHELP.HEART, restricted to non-smokers (n=2501, selection criterion: smoking_status=:'N'), but strive for a distribution of blood pressure status (BP_Status) as in the unrestricted dataset, i.e. (PROC FREQ output):

                    Blood Pressure Status

                                      Cumulative    Cumulative
BP_Status    Frequency     Percent     Frequency      Percent
--------------------------------------------------------------
High             2267       43.52          2267        43.52
Normal           2143       41.14          4410        84.66
Optimal           799       15.34          5209       100.00

/* Store stratum allocation proportions in a dataset STRATA */

proc freq data=sashelp.heart noprint;
tables bp_status / out=strata(drop=count rename=(percent=_alloc_));
run;

/* Select the subset (input dataset for PROC SURVEYSELECT) */

proc sort data=sashelp.heart out=restrpop;
where smoking_status=:'N';
by bp_status;
run;

/* Draw the random sample */

proc surveyselect data=restrpop
method=srs rate=0.8
seed=2718 out=want;
strata bp_status / alloc=strata;
run;

/* Check distribution of BP_Status */

proc freq data=want;
tables bp_status;
run;

Result:

                    Blood Pressure Status

                                      Cumulative    Cumulative
BP_Status    Frequency     Percent     Frequency      Percent
--------------------------------------------------------------
High              871       43.53           871        43.53
Normal            823       41.13          1694        84.66
Optimal           307       15.34          2001       100.00

Note that, e.g., with rate=0.9 the stratum sample size for stratum BP_Status='Optimal' would (necessarily) be capped at the stratum total, leading to not strictly proportional sample size allocation (see notes in the SAS log in this case).

Re: Creating a random Sample under constraint

fedeava — Fri, 08 May 2020 13:16:33 GMT

Wonderful!

This was exactly the help I was looking for. I was able to create a stratified sample on 3 drivers.... I'll probably have to add further drivers... I hope it continues to perform..

Thanks again!