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Re: st: svy + aweights


From   Joerg Luedicke <joerg.luedicke@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: svy + aweights
Date   Thu, 10 Nov 2011 16:28:58 -0500

> I would be downweighting the subjects with fewer days;

Whether you downweight subjects with fewer days or upweight subjects
with more days should be the same thing. Anyway, if your concern is
that of differing reliability across sub-groups of your data, then
weighting will not solve the problem.

> I wouldn't actually use number of days but the inverse of the variance
> of the daily average, since I have that, number of days is just a short
> hand way of thinking about it.

I am completely at a loss with this one.

>
> Not all obese kids have less days than normal kids, it's just that
> the distribution is skewed down - higher percent with 1-2 days and
> lower percent with 6-7 days.

As I said, consider a multilevel model. That way you can directly
account for the uncertainty that is related to the varying numbers of
measurement occasions.

J.

>
> On 11/10/2011 3:37 PM, Joerg Luedicke wrote:
>>
>> Then it seems to be a problem of reliability of your measure, i.e.,
>> measurement for obese kids is less reliable than measurement for
>> non-obese kids, right? Now, if you upweight the obese kids in your
>> sample, why would that enhance the reliability of their measurement?
>> If I understand the problem correctly, then weighting strikes me as
>> the wrong approach here.
>>
>> Perhaps you could consider not averaging at all and running a
>> multilevel model of some sort.
>>
>> J.
>>
>> On Thu, Nov 10, 2011 at 3:23 PM, Jeph Herrin<stata@spandrel.net>  wrote:
>>>
>>> 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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