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From |
"Tim Wade" <wadetj@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: lincom & svy commands |

Date |
Thu, 23 Oct 2008 15:24:04 -0400 |

Jacqui, here is an example that probably does not make any sense but illustrates what lincom does: . clear . webuse nhanes2f . svyset psuid [pweight=finalwgt], strata(stratid) pweight: finalwgt VCE: linearized Single unit: missing Strata 1: stratid SU 1: psuid FPC 1: <zero> . svy: mean zinc leadwt (running mean on estimation sample) Survey: Mean estimation Number of strata = 31 Number of obs = 9189 Number of PSUs = 62 Population size = 104176071 Design df = 31 -------------------------------------------------------------- | Linearized | Mean Std. Err. [95% Conf. Interval] -------------+------------------------------------------------ zinc | 87.18207 .4944827 86.17356 88.19057 leadwt | 15726.67 340.4806 15032.25 16421.08 -------------------------------------------------------------- . lincom zinc-leadwt ( 1) zinc - leadwt = 0 ------------------------------------------------------------------------------ | Coef. Std. Err. t P>|t| [95% Conf. Interval] -------------+---------------------------------------------------------------- (1) | -15639.49 340.5765 -45.92 0.000 -16334.1 -14944.88 ------------------------------------------------------------------------------ . matlist e(b) | zinc leadwt -------------+---------------------- y1 | 87.18207 15726.67 . matlist e(V) | zinc leadwt -------------+---------------------- zinc | .2445131 leadwt | -32.53403 115927 . *standard error from covariance matrix . di sqrt((v[1,1])+(v[2,2])-(2*v[2,1])) 340.57646 . scalar se=sqrt((v[1,1])+(v[2,2])-(2*v[2,1])) . *t statistic . di r(estimate)/se -15639.486 hope this helps, Tim On Thu, Oct 23, 2008 at 11:18 AM, Bell, Jacqueline S. <j.bell@abdn.ac.uk> wrote: > Hi > > This is a follow-on to a previous message I sent this month asking about how lincom calculates standard errors when clustering is present. > > Can anyone advise me on what lincom actually does when estimating differences in parameters from svy:mean or svy:prop? > > I have before/after data which is not paired at an individual level, but has a cluster structure. In these circumstances it is not obvious how lincom goes about estimating the before/after difference. > The two alternatives suggested to me are: > i) it estimates before and after separately for the whole population, then estimates the difference > ii)it estimates the difference in each cluster, and then the overall difference. > > In the data there are (in most cases) before and after data for each cluster, but often quite severe imbalances in samples before/after within cluster. > > > Thanks for any help, Jacqui > > > > The University of Aberdeen is a charity registered in Scotland, No SC013683. > > * > * 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/ > * * 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/

**References**:**st: lincom & svy commands***From:*"Bell, Jacqueline S." <j.bell@abdn.ac.uk>

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