Happy holidays to all! At the recent FCSM conference I was discussant at a session on missing data. The main thrust of my discussion was that multiple imputation was underutilized by survey producers as a method for handling item nonresponse in surveys, given that Rubin proposed the original idea over 30 years ago and software is now much more widely available. Fritz Scheuren thought the discussion might stimulate an interesting debate, and suggested that I post the discussion on the SRMS list serve. It is attached to this message in pdf form, and can also be accessed on my own web site at
http://sitemaker.umich.edu/rlittle/files/fcsmdisc.pdf Some of my remarks are specific to the papers in the session, and hence depend on context, but I welcome any comments! An important issue in survey nonresponse adjustments is how to deal with the design variables, and in particular the role of the sampling weights. A common approach for unit nonresponse is to multiply the sampling weight for each respondent by the nonresponse weight, computed within adjustment cells as the sum of the sampling weights for respondents and nonrespondents divided by the sum of the sampling weights for respondents. In the imputation setting this corresponds to imputing nonrespondent values by the sample-weighted respondent mean in the adjustment cell. In a paper recently published in Statistics in Medicine, "On weighting the rates in nonresponse weights", Sonya Vartivarian and I show by simulations that this approach is generally biased when the survey outcome is related to the design variables. The correct approach is to include the design variables as covariates when creating adjustment cells. A draft of this paper can be accessed at http://sitemaker.umich.edu/rlittle/files/vartivrev.pdf If too many cells result with this approach, they can be reduced by methods such as those described in my 2002 and 2003 JSM SRM Proceedings papers with Sonya. These papers are available from my web site http://sitemaker.umich.edu/rlittle by clicking on the "link to download recent papers". Rod Little References: Little, R.J. and Vartivarian, S. (2003). On weighting the rates in nonresponse weights. Statistics in Medicine 22, 1589-1599. Vartivarian, S. and Little, R.J.A. (2002). On the Formation of Weighting Adjustment Cells for Unit Nonresponse. American Statistical Association 2002, Proceedings of the Survey Research Methods Section, 3553-3558. Vartivarian, S. and Little, R.J. A. (2003). Weighting adjustments for unit nonresponse with Multiple outcome variables.To appear in American Statistical Association 2003, Proceedings of the Survey Research Methods Section. ___________________________________________________________________________________ Roderick Little Richard D. Remington Collegiate Professor (734) 936-1003 Department of Biostatistics Fax: (734) 763-2215 U-M School of Public Health M4045 SPH II [EMAIL PROTECTED] 1420 Washington Hgts http://www.sph.umich.edu/~rlittle/ Ann Arbor, MI 48109-2029
fcsmdisc.pdf
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