[R] longitudinal survey data
Thomas Lumley
tlumley at u.washington.edu
Sat May 28 01:35:25 CEST 2005
On Fri, 27 May 2005 h.brunschwig at utoronto.ca wrote:
> Sorry, still confused. If I dont have fpc's ready in my dataset (calculate
> myself?) that means that R will use the weight of an individual for each of his
> repeated observations. But is that then still correct? The "cluster" individual
> is ignored and each observation of an individual has the same weight.
>
Well, it depends to some extent on what inferences you are making, but
yes, you probably do want each observation to have the same weight.
Suppose you have 4 measurements on each person, and you are working with a
simple random sample of 1000 people from a population of 1,000,000. If you
had done these 4 measurements on the whole population you would have
4,000,000 measurements, so the 4000 measurements you have are 1/1000 of
the population. This is the same weighting as if you had a single
measurement person person, giving 1000 measurements in the sample and
1,000,000 in the population.
If different individuals have different numbers of measurements then
things get a bit trickier. It depends then on why there are different
numbers of measurements.If they are the result of non-response you might
want to rescale the weights at later time points to give the right
population totals. If they are part of the sampling design then the
design will specify what to do with them.
-thomas
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