# Re: st: trend analysis for survey data

 From Steven Samuels To statalist@hsphsun2.harvard.edu Subject Re: st: trend analysis for survey data Date Mon, 10 Mar 2008 15:03:26 -0400

The mean of a binary variable is the proportion with the event. Thus - svy regression- will give a test for trend in proportions. For proportions between 20% and 80%, linear, logit, and probit transformations have similar fits (DR Cox, Analysis of Binary Data, Methuen 1970). The 'Chi Square Test for Trend in Proportions' for independent, non-survey data (e.g. PA Armitage Statistical Methods in Medical Research,Wiley, 1971, p 363) is essentially a test for a non- zero linear regression coefficient for proportions under the null hypothesis. (Moreover, it is the maximum likelihood score test for trend in logistic regression (reference: I forget).) All in all the linear version is much easier to interpret. Either the linear or logistic version have the proper type 1 error rate, but Audrey should decide which scale to use after plotting her data. Certainly if she wants to interpolate, extrapolate, or summarize the prediction with a regression coefficient, she should choose a well-fitting model.

-Steven

On Mar 10, 2008, at 2:35 PM, Mohammed El Faramawi wrote:

```Hi steven,
It seems that I am missing something. Why do you
recommend using svy:regress with a categorical
variable?? why not svy:logit from the beginning?
thank oyu

```
```-svyset- your data. Then try -svy reg- for your 0-1
outcome on the
time dimension variable.  If the prevalences are
close to zero or
one, you could consider -svy logit -.  From your
question, I take it
that the surveys are independent, but taken over the
time dimension
and that the target populations were identical,
except for time
period.  You will need to define new strata, which
are combinations
of survey and the original strata.

This assumes that primary sampling units were newly
sampled for each
survey. If not, you cannot treat the surveys as
independent and
should come back here for help.

-Steven
On Mar 10, 2008, at 4:28 AM, audreyp@neo013.net
wrote:

```
```hi,
I have data on three/four independent
```
```cross-sectional surveys (not
```
```panels). I need to test for a trend in the
```
```prevalence of a health
```
```condition. How can I do a trend analysis in Stata
```
```that will take
```
```into account sampling weights and stratification?
thank you.

Audrey P.

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```Steven Samuels
845-246-0774
18 Cantine's Island
Saugerties, NY 12477
EFax: 208-498-7441

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