Thanks Steve for enlightening!
hfd--
----- Original Message -----
From: Steven Samuels <[email protected]>
Date: Monday, September 22, 2008 0:22 am
Subject: Re: st: Median test & ANOVA with sampling weights
> I meant to finish the sentence in the second paragraph:
>
> "This is because the mean for the population will be closer to
> that
> of the diabetics because it contains a contribution from the
> diabetics."
> -Steve
>
> On Sep 21, 2008, at 12:53 PM, Steven Samuels wrote:
>
> > hafida--
> >
> > 1. "Since it's not a case control study, I thought that
> comparing
> > those with and without diabetes was inappropriate"
> >
> > That's not correct. You want to compare diabetics to the whole
> > population. This is *equivalent* to comparing diabetics to non-
> > diabetics. There is no stata command which compare part of a
> > sample with the whole sample, but there are plenty (-cendif-) is
>
> > one, which will compare a part to the other part and give you a
> CI
> > for the difference.
> >
> > This is easiest to illustrate with means: Suppose the mean for
> > diabetics for a variable is 10 and that for non-diabetics is 10.
>
> > The difference is zero. If diabetics are 10% of the population,
>
> > the mean for the population is (.1 x 10) + (.9 x 10) = 1 + 9 =
> > 10. The difference between this and the diabetics' mean is also
>
> > zero. On the other hand, suppose that the mean for non-
> diabetics
> > is 20; the difference from the mean of the diabetics is 10.
> Then
> > the population mean is .1 x 10 + .9 x 20 = 1 + 18 =19; the
> > difference from the mean of diabetics is 9. Notice that the
> > diabetic/population difference is < diabetic/non-diabetic
> > difference. This is because the d
> >
> >
> > 2. As -cendif- is a rank procedure, you will get the same
> results
> > for any transformation. There is no need to transform.
> >
> > 3. If you are uncertain of basic math functions, it is time to
> > review; you will not be happy in epidemiology without a working
> > knowledge of back-transformations. To answer your question
> about
> > the "cubic": x^3 and x^(1/3) are inverses in Stata (-help
> > operators-). Not sure what this means? try a google search on:
> > inverse function introduction.
> >
> > I strongly suggest that you consult a Biostatistics staff member
> at
> > Newcastle.
> >
> > Good luck!
> >
> > -Steve
> >
> >
> >
> >
> > On Sep 19, 2008, at 11:06 PM,
> > [email protected] wrote:
> >
> >> Hi Steve and all,
> >> I think you're correctly recognising my situation: I might have
>
> >> taken the sampling issue wrong so far.
> >> For additional information, I'm working with a data set from a
> >> national longitudinal survey with three age cohorts (young,
> mids,
> >> older) which were randomly re-sampled from Medicare database
> >> employing stratified random sampling.
> >>
> >> . svyset [pweight=o1wtarea], strata(o4state)
> >> pweight: o1wtarea
> >> VCE: linearized
> >> Single unit: missing
> >> Strata 1: o4state
> >> SU 1: <observations>
> >> FPC 1: <zero>
> >>
> >> I focus on older cohort only at a certain time point (4th
> survey)
> >> and my sample is those with diabetes. My project aims to look
> at
> >> if different patterns of cardiovascular medication use is
> >> associated with quality of life (4 dimensions of SF-36). The
> study
> >> design is pretty simple, cross sectional. However, I have
> received
> >> some input that comparison between my sample and the entire in
> the
> >> cohort (older at survey 4) is worth performing. Since it's not
> a
> >> case control study, I thought that comparing those with and
> >> without diabetes was inappropriate, leading me to consider
> using -
> >> svy- (which maybe equally or even more inappropriate!). Your
> >> suggestion, however, indicates that my previous thought was ok
> and
> >> I perhaps needn't use -svy- at all. Did I take it correctly?
> >>
> >> Some of the dependent variables are skewed and -gladder- offers
>
> >> cubic transformation to best approximate normal distribution.
> If
> >> any median test is not fairly robust, is comparing transformed
> >> means acceptable in this case? (My concern is that cubic
> >> transformation, perhaps unlike log, will inflate type I error).
>
> >> Also, what is the command to perform a back transformation from
>
> >> cubic? (I'm definitely not a maths nerd :)).
> >>
> >>
> >> thanks,
> >> hafida--
> >>
> >>
> >> On Sep 20, 2008, at 1:11 AM Steven Samuels to statalist wrote:
> >>
> >> hafida--
> >>
> >> You've given us very little information about your survey
> sample
> >> and its design. More would have been helpful.
> >>
> >> You appear to be misusing the terms "sample" and "population".
> A
> >> "population" is the larger group of people represented by the
> >> sample; statistics for a population are known from outside
> sources
> >> such as a census. For example, in the U.S. a sample of 1500
> people
> >> might represent the population of millions. What you are
> calling
> >> "sample" and "population" appear to be, respectively, one
> >> subgroup of a sample (those with dmstat=1) and the entire sample.
> >>
> >> The proper way to compare one subgroup to the whole group is to
>
> >> compare the subgroup to the others. So, form two groups: group
> = 1
> >> if dmstat =1 and group = 2 if dmstat is not 1 (the rest of the
> >> sample).
> >>
> >> -pctile- will estimate weighted medians, but the CI's will not
> be
> >> correct, for they assume independent observations. To proceed,
> you
> >> must know the sampling design, including cluster and stratum
> >> information. The program -cendif- by Roger Newson (-findit
> >> cendif-) will estimate differences in the medians and
> accommodates
> >> sampling weights and clustering. The sign test, in contrast, is
>
> >> for a set of paired independent observations, not for any list
> of
> >> paired numbers.
> >>
> >> To do ANOVA, you must first -svyset- your data and use -svy:
> reg-.
> >> There is nothing special about -svy: reg-; ust set up the ANOVA
> as
> >> you would do with ordinary -reg-. To compare individual groups
> to
> >> one another, after the regression run -test-, with options -
> mtest
> >> (holm)- or -mtest(sidak)-.
> >>
> >> Your post shows that you are fairly new to sampling concepts.
> >> Before proceeding, I suggest that you look at a good text; I
> >> recommend "Sampling Design and Analysis", by Sharon Lohr. Your
>
> >> faculty may be able to suggest local resources.
> >>
> >> -Steve
> >>
> >>
> >> On Sep 19, 2008, at 7:53 AM,
> >> [email protected] wrote:
> >>
> >>
> >> I'm using a survey data and wonder how can I perform a
> >> comparison between median in the sample and in the population.
> >> Medians were separately obtained using -pctile- or -_pctile-.
> >>
> >> . pctile pctGH = o4gh [pw=o1wtarea], nq(4) genp(percent)
> >> . list percent pct in 1/4
> >> +-----------------+
> >> | percent pctGH |
> >> |-----------------|
> >> 1. | 25 50 |
> >> 2. | 50 67 |
> >> 3. | 75 77 |
> >> 4. | . . |
> >> +-----------------+
> >>
> >> . pctile pctileGH1 = o4gh if dmstat==1 [pw=o1wtarea], nq(4)
>
> >> genp(pctGH1)
> >> . list pctGH1 pctileGH1 in 1/4
> >> +------------------+
> >> | pctGH1 pctileGH1 |
> >> |------------------|
> >> 1. | 25 40 |
> >> 2. | 50 60 |
> >> 3. | 75 72 |
> >> 4. | . . |
> >> +------------------+
> >>
> >> Should I calculate the difference between each value in the
>
> >> sample and population first and carry out a sign test then? If
> so,
> >> how is sampling weight taken into account? (I mean, can I use
> >> weighted median in the population to substract each
> 'unweighted'
> >> value?)
> >>
> >> Secondly, is it possible to perform one-way ANOVA with
> >> sampling weight, particularly for post-hoc comparison? Using
> svy:
> >> regress did not give enough information.
> >> *
> >> * For searches and help try:
> >> * http://www.stata.com/help.cgi?search
> >> * http://www.stata.com/support/statalist/faq
> >> * http://www.ats.ucla.edu/stat/stata/
> >
> > *
> > * For searches and help try:
> > * http://www.stata.com/help.cgi?search
> > * http://www.stata.com/support/statalist/faq
> > * http://www.ats.ucla.edu/stat/stata/
>
> *
> * For searches and help try:
> * http://www.stata.com/help.cgi?search
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
>
*
* For searches and help try:
* http://www.stata.com/help.cgi?search
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
