Han-Lin I don't think I have seen a reply so I will suggest that maybe you could try a different approach than what you are thinking about doing. I believe the current best practice is to use the weights as a covariate in a regression model - and bytheway - the weights are the inverse of the probabilities of selection - not the probabilities.
Fundamentally, there is a difficulty in making sense out of 'random effects' in a finite population setting. (plagiarized from some unknown source) See: < 9. Pfeffermann, D. , Skinner, C. J. , Holmes, D. J. , Goldstein, H. , and Rasbash, J. (1998), ``Weighting for unequal selection probabilities in multilevel models (Disc: p41-56)'', Journal of the Royal Statistical Society, Series B, Methodological, 60 , 23-40 > which refers back to: <29. Pfeffermann, D. , and LaVange, L. (1989), ``Regression models for stratified multi-stage cluster samples'', Analysis of Complex Surveys, 237-260 > If you don't like statistical papers, then see section 4.5 of <8. Korn, Edward Lee , and Graubard, Barry I. (1999), ``Analysis of health surveys'', John Wiley & Sons (New York; Chichester) > They explain the idea of using weights in a model fairly simply. Bob -----Original Message----- From: Han-Lin Lai [mailto:[EMAIL PROTECTED] Sent: Wednesday, May 19, 2004 12:47 PM To: [EMAIL PROTECTED] Subject: [R] mixed models for analyzing survey data with unequal selection probability Hi, I need the help on this topic because this is out of my statistical trianing as biologist. Here is my brief description of the problem. I have a survey that VESSELs are selected at random with the probability of p(j). Then the tows within the jth VESSEL are sampled at random with probability of p(i|j). I write my model as y = XB + Zb + e where XB is fixed part, Zb is for random effect (VESSEL) and e is within-vessel error. I feel that I should weight the Zb part by p(j) and the e-part by p(i,j)=p(j)*p(i|j). Is this a correct weighting? How can I implement the weightings in nlme (or lme)? I think that p(i,j) can be specified by nlme(..., weights=p(i,j),...)? Where is p(j) to be used in nlme? I appreciate anyone can provide examples and literature for this problem. Cheers! Han ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
