Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.

# Re: st: sampling query

 From Stas Kolenikov <[email protected]> To [email protected] Subject Re: st: sampling query Date Thu, 27 Jan 2011 13:37:51 -0600

```Rich,

Look up "multiple frames". That's a more common term for samples in
which the ultimate unit can be reached through different trajectories
(say landline phone sample, cell phone sample, and area/personal visit
sample). The probabilities should be combined as

1 - Prob[ in the sample ] = product over k of (1-Prob[ reach the unit
through the k-th frame ] )

which for small probabilities leads to sum of selection probabilities.
You are totally right that the probability should go up rather than
down.

On Thu, Jan 27, 2011 at 10:37 AM, Richard Goldstein
<[email protected]> wrote:
> all,
>
> I have received a report in which the report writer was stuck with the
> following design (already implemented before his involvement): a number
> of "cohorts" were set up (22 of them in fact) and the definitions of
> these cohorts were not mutually exclusive (i.e., there was some overlap
> in membership so that a given observation could appear in more than 1
> cohort); to calculate the probability weights, the report writer first
> calculated the probability of inclusion for each cohort (simply as n/N
> where n is sample size from cohort and N is population size of cohort).
>
> For observations in more than one cohort, who were actually selected, he
> then multiplied the inclusion probabilities of each cohort that
> observations was in. Since each inclusion probability is less than 1,
> the combined inclusion probability is smaller than the individual
> inclusion probabilities for the individual cohort. And then, of course,
> the weights are greater for these people (since the weight is just the
> inverse of the inclusion probability).
>
> However, since these observations are in more than one cohort, shouldn't
> the combined probability be greater for them (rather than smaller)?
>
> How should the combined inclusion probability be calculated?
>
> Or am I just wrong and the writer of the report is correct?
>
> Any references on dealing with overlapping "cohorts" would also be
> greatly appreciated.
>
> Rich
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>

--
Stas Kolenikov, also found at http://stas.kolenikov.name
Small print: I use this email account for mailing lists only.

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/
```