Re: st: svy + aweights

[Jeph Herrin <stata@spandrel.net>]

For each day, I have 1440 minutes (24 hours) of measurements. Each minute
has an activity measure, 0-30,000. I want to compare how active the kids
(these are all children) are, so I calculate an activity measurement for
each day (to keep it simple here I will say it is the median, though actually
it is a complicated function of the activity levels over the day).


id   day1 day2 day3   obese  average  days
1    500   500  500     Y      500      3
2    1000               N     1000      1


Now I want to compare kids who are normal weight to those who are
obese. It turns out, I don't have as many measurements on the obese
kids because they did not wear their monitor as often. So the
active kids have more precise daily averages than the obese kids.
To compare average activity, I want to account for the differences
in precision.

If this was not -svy- data, I would use something like

   ttest average [aw=days], by(obese)

even better -reshape- the data to have one record per day per id and use

  xtset id
  xtreg average

But here I have this complex survey design to deal with.

thanks,
Jeph

On 11/10/2011 3:06 PM, Joerg Luedicke wrote:
> I do not quite understand what you are trying to do. Suppose we have
> two individuals, one measured only once and the other on, say, 3
> occasions. Let's further assume that activity is measured in minutes
> (btw, how is your dependent variable measured?). We could have the
> following data:
>
> id day1 day2 day3
> 1  30
> 2  10  10  10
>
> If you calculate the minutes per day now (whether or not this being a
> proper way of handling it), id#1 will end up with 30 and id#2 with 10
> minutes. I do not understand why id#2 is supposed to weigh more than
> id#1?
>
> J.
>
>
> On Thu, Nov 10, 2011 at 2:34 PM, Jeph Herrin<stata@spandrel.net>  wrote:
> > Thanks for the suggestion, but I specifically need to give more
> > weight to subjects which have more days of observation. For example,
> > I have
> >
> >    svy : regress activity female BMI
> >
> > and would like this regression to give more weight to subjects which
> > have more days of observation. Using activity/days as the dependent
> > variable will not do this.
> >
> > Jeph
> >
> > On 11/10/2011 1:58 PM, Stas Kolenikov wrote:
> > > Rather than forming the mean activity per day, you might want to
> > > analyze this as a ratio:
> > >
> > > svy : ratio activity / day_reported
> > >
> > > or whatever would be an appropriate ratio. That way, you will get
> > > correct standard errors without messing with the analytical weights.
> > >
> > > On Thu, Nov 10, 2011 at 1:46 PM, Jeph Herrin<stata@spandrel.net>    wrote:
> > > > I am analyzing NHANES data (see manual page for -svyset-) using -svy-
> > > > commands. My complication is that I am using the subset of subjects for
> > > > which there is activity monitoring, and the number of days monitored
> > > > varies
> > > > from 1 to 8. So - to be clear - for some subjects I have 1 day of
> > > > monitoring,
> > > > and for some I have 2 days, some I have 3, etc. My dependent variable of
> > > > interest is daily average activity levels, but I would like this to be
> > > > weighted by the number of days monitored. (This is important because
> > > > there
> > > > seems to be a clear relationship between days monitored and age, race,
> > > > etc).
> > > >
> > > > How do I incorporate this additional level of weighting? For instance,
> > > > if I use
> > > >
> > > >   svy : mean depvar [aw=days]
> > > >
> > > > I get an error that weights are not reported.
> > > >
> > > > thanks,
> > > > Jeph
> > > > *
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