## Support #307

### Survey design xwaves

100%

**Description**

Hi, I have constructed a confidence interval for a linear combination of point estimates across BHPS waves 1, 2 and 3. Specifically, I have calculated the share of mortgagors in arrears in each year, and then taken a simple average over these years. I would like to check my methodology for constructing the confidence interval.

First I have only specified the cluster variable psu in my survey design, rather than the strata, to allow for correlation between clusters across years. I have also used xhwght for each year but am worried this is incorrect?

Then I have used the command in stata:

svy, subpop(subpop): mean arrears, over(year)

where subpop is those with tenure==2, arrears is a 1/0 variable, and year takes the values 1991, 1992 or 1993.

I then use the lincom command to take a simple average across the point estimate from each year, which gives me confidence intervals.

Is this methodology correct? Should I be using different weights?

Thanks

Daisy