Biostatistics and research methods

Biostatistics and research methods
The following questions are based on two papers relating to the use of Xylitol for caries prevention.
“Results from the Xylitol for Adult Caries Trial (X-ACT)”, JADA 2013, 144 (1), p21–30.
“Design of the xylitol for adult caries trial (X-ACT)”. BMC Oral Health 2010;10:22.
They can be found from the link below:
http://jada.ada.org/article/S0002-8177(14)60565-1/pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2955560/pdf/1472-6831-10-22.pdf
1. Study Design:
1.1. Briefly state the type of study/study design used in the X-ACT study. [2] 1.2. From a research terminology view point, is it an experimental/observational, prospective/retrospective, cross-sectional/longitudinal study? Please justify your
statement. [3] 1.3. Describe in your own words the research question the authors aimed to answer. [2] 1.4. Briefly describe a different study design (other than a randomized clinical trial) to study the relationship between a potential active agent (e.g. Xylitol or
Fluoride) and caries. [2]

2. Outcome:
2.1. What is the primary outcome measure used in this study? [1] 2.2. Please identify what type of variable D2FS is. What values can the D2FS variable take? Name the statistical model used in the paper to model D2FS incidences?
Why do you think the authors chose such a model over the more familiar z-test or t-test or test based on two proportions? Can you identify another model in the course
notes that might be used for such data? [5] 2.3. For simplicity, let’s assume D2FS is a continuous variable. Please suggest a statistical model (distribution) that has been covered in our course to model such
data. To justify your choice of model, one may plot the histogram of D2FS. What features on the histogram will you check to justify your choice? What type of
descriptive statistics for central tendency and spread will you use to summarize the D2FS? [3]

3. In most dental/medical research papers, a baseline characteristics table such as Table 1 in the article is displayed.
3.1. From Bader et. al.(2003) Table 1, please identify the following variables’ data types. State the choice of descriptive statistics and potential plots/figures
you may use to explore these variables: “Race or Ethnicity”; “Age in Years”; “Brushes Two or More Times per Day”. [3] 3.2. In Table 1, the authors report the baseline age in years for all study subjects (under TOTAL column) to be 47 with a standard deviation of 13.6. Please explain
the differences between standard deviation and standard errors of the mean. [2] 3.3. Assume Age is normally distributed. Calculate the age range that 95% of the study subjects (roughly around 647 patients among 681) fall into. [2] 3.4. Propose a range of ages with 90% confidence that the true mean age of the study population will fall into. [2] 3.5. In Table 1, it is reported that 31.6% of the study population (high risk of caries) routinely have dental visits. Assume in a national survey study it is
reported that 37% of the general population have routine dental visits. Does the study population in this paper have a different dental visit habit from the general
population? Please write down the null and alternative hypotheses, conduct the analysis, and write a short report. [3] 3.6. Provide reasons why it is important to have such baseline characteristics table in any medical/dental research. What will you conclude from Table 1? [2]

4. Results/Conclusions:
4.1. In Bader et al.(2013), what is the annualized D2FS increment reported in each group? What are the measures and the reported values the authors used to compare
these two numbers? Does xylitol have any effect? Explain your conclusion. [3] 4.2. Discuss the difference between clinical importance and statistical significance. In clinical terms, what does a 10% reduction in increment mean while using
Xylitol? What percentage of reduction did the authors have in mind to make the use of Xylitol to be clinically meaningful? [3] 4.3. Assume a policy maker read this article; decided it was a well-conducted study by a group of highly regarded researchers. He therefore concluded “XYLITOL is
not working in preventing caries and should not be used on the market”. Is it a fair conclusion to make? (Think of the limitations of the study)

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