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Unformatted text preview: PUBH 7430 Assignment 3 Solutions October 18, 2011 Due: October 27, 2011 at the beginning of class On this assignment, as in all homeworks, UNEDITED COMPUTER OUTPUT IS NOT ACCEPTABLE . Tables and plots prepared from statistical output should be appro- priate for inclusion in a scientific report, with all statistics rounded to a reasonable number of significant digits. Background For this assignment, you will be using the same dataset as in Assignment 2. The description is provided below. 151 persons with gum disease (elevated oral MS levels) were recruited into a study at a mid-western research university and randomly assigned to one of three treatment groups. The treatment under study was a gum with an active drug (Xylitol). There were two control groups: one group received gum with Sorbitol but no Xylitol and one group received no gum. Participants in the gum groups were asked to chew the gum three times daily for a minimum of 5 minutes each time and carry out their usual oral hygiene (tooth brushing, mouthwash, etc.). Participants in the group without gum were asked to carry out their usual oral hygiene only. For this assignment, we will focus only on the medicated and placebo gum groups, omitting the no gum group. Treatment group is recorded as Drug or Placebo. 1 Subjects rinsed their mouths twice daily for 14 days with a 0.12% CHZ gluconate mouthrinse. They were then asked to follow their assigned treatment for three months. Participants had their oral levels of mutans streptococci measured four times: at baseline after the 14 days of CHZ gluconate mouthrinse (after which they were randomly allocated to one of the three treatment groups), one week later, 1 month later, and 3 months later. The four mut* variables are the levels of oral mutans streptococci at the four time periods in units of log(cfu/ml) (where cfu = colony forming units, a count of blotches on a standard sized petri dish after standard preparation). Missing MS levels are recorded as NA in the R data file and . in the SAS dat file. For this assignment, we will not make use of the other covariates in this dataset: age, race, gender, and general condition of the teeth. This latter is measured by the variables dmft and dmfs. The variable dmft stands for diseased/missing/filled teeth and dmfs stands for diseased/missing/filled surfaces. (There are 5 surfaces to a tooth, the four sides and the chewing surface.) Questions Suppose we are interested in knowing how the effect of treatment changes over time. We will continue to treat mutpre as a baseline variable, so our correlated outcomes of interest are mutpost, mut1mos and mut3mos . 1. Linear predictor. Construct (and write down) a linear predictor which will allow you to assess whether, given baseline MS levels, the effect of treatment changes over time (hint: you will want to use an interaction term)....
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This note was uploaded on 11/21/2011 for the course PUBH 7430 taught by Professor Prof.eberly during the Fall '04 term at Minnesota.
- Fall '04