Re: [R] longitudinal survey data

From: Thomas Lumley <tlumley_at_u.washington.edu>
Date: Sat 28 May 2005 - 09:35:25 EST

On Fri, 27 May 2005 h.brunschwig@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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