Date: Sun, 9 Nov 2008 20:23:11 -0500 David Greenberg "SPSSX(r) Discussion" David Greenberg Re: evaluating differences in count data To: Marta García-Granero <491568B7.1060808@gmail.com> text/plain; charset=iso-8859-1

I think this test is not appropriate. It is the test that would be appropriate if the data were being generated by a Poisson distribution. That was my own initial assumption when I responded to the initial inquiry. However, for each individual the response is binary. The assumption of independence of events that is characteristic of a Poisson distribution does not hold here. David Greenberg, Sociology Department, New York University

----- Original Message ----- From: Marta García-Granero <mgarciagranero@gmail.com> Date: Saturday, November 8, 2008 5:25 am Subject: Re: evaluating differences in count data To: SPSSX-L@LISTSERV.UGA.EDU

> Pirritano, Matthew wrote: > > Sorry, I did leave a little info out. The intervention was intended > to > > increase the use of preventive medical procedures. The increased use > was > > encouraged by rewarding their use with monetary incentives. The > > intervention also provided money to increase the population size > > overall, which contains a diabetic population. Lipid assays are a > > preventive measure for diabetics. So, we'd expect the number of lipid > > assays to go up if doctors are taking advantage of the monetary > > incentives. Hence the two hypothetical counts I gave below. The number > > of lipid assays went up, but is increase greater than would be expected > > if the intervention had no effect ( = the null hypothesis). > > > > See chapter 6 of Statistics at Square One (section "Standard error of > a > total") > http://www.bmj.com/collections/statsbk/6.dtl > > They describe a simple method to compare two counts. Using your data: > > * Standard error of a total (& difference between two totals) *. > DATA LIST LIST/n1 n2 (2 F8.0). > BEGIN DATA > 280 365 > END DATA. > COMPUTE zvalue = (n1-n2)/SQRT(n1+n2). > COMPUTE pvalue = 2*(1-CDF.NORMAL(ABS(zvalue),0,1)). > LIST. > > HTH, > Marta García-Granero > > > > > Thanks > > matt > > > > Matthew Pirritano, Ph.D. > > Research Analyst IV > > Medical Services Initiative (MSI) > > Orange County Health Care Agency > > (714) 834-3566 > > -----Original Message----- > > From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On > Behalf Of > > Gene Maguin > > Sent: Thursday, November 06, 2008 12:19 PM > > To: SPSSX-L@LISTSERV.UGA.EDU > > Subject: Re: evaluating differences in count data > > > > Matthew, > > > > > >>> I'm trying to find out if there is a way to do inferential tests on > >>> > > count > > data. I am comparing the number of individuals who have had one lipid > > assay > > in the year prior to an intervention with the number of individuals > who > > have > > had one lipid assay and in the year after the intervention. So I have > > two > > numbers which are counts. Let's say 280 individuals with one lipid assay > > prior to the intervention and 365 individuals with an assay after the > > intervention. > > > > > > I'm confused by your description. Are you saying that you had a pool > of > > people (say, N=500) who were 'assigned' to a pre-intervention lipid > > assay, > > an intervention and a post-intervention lipid assay? 280 of 500 did > the > > pre-intervention assay and 365 of the 500 did the post-intervention > > assay? > > And you want to compare the proportion who completed the pretest to > the > > proportion who completed the posttest? > > > > Or, do I misunderstand and you are interested in something else? > > > > Gene Maguin > > > > ===================== > > To manage your subscription to SPSSX-L, send a message to > > LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except > the > > command. To leave the list, send the command > > SIGNOFF SPSSX-L > > For a list of commands to manage subscriptions, send the command > > INFO REFCARD > > > > ===================== > > To manage your subscription to SPSSX-L, send a message to > > LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except > the > > command. To leave the list, send the command > > SIGNOFF SPSSX-L > > For a list of commands to manage subscriptions, send the command > > INFO REFCARD > > > > > > > -- > For miscellaneous statistical stuff, visit: > http://gjyp.nl/marta/ > > ===================== > To manage your subscription to SPSSX-L, send a message to > LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except the > command. To leave the list, send the command > SIGNOFF SPSSX-L > For a list of commands to manage subscriptions, send the command > INFO REFCARD

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