A case-control study is being designed to look at the relationship between epilepsy and a new vaccine for varicella.
What is the usual outcome measure in a case-control study?
Correct Answer B:
Case-control studies - odds ratio.
Study design:
The following table highlights the main features of the main types of study:
A clinical trial is conducted to study the benefits of a new oral medication to improve the symptoms of patients with chronic obstructive pulmonary disease (COPD). In the trial 300 patients with COPD are given the new medication and a further 300 COPD patients are given a placebo. Three months later they are asked to rate their symptoms using the following scale: much improved, slight improvement, no change, slight worsening, significantly worse.
What is the most appropriate statistical test to see whether the new medication is beneficial?
Correct Answer A:
The first point to note is that the outcome measure is not normally distributed, i.e. it is non parametric. This excludes the Student's t-tests. We are not comparing percentages/proportions so the chi-squared test is excluded.
The Wilcoxon signed-rank test is used to compares two sets of observations on a single sample or matched samples.
Significance tests: types:
The type of significance test used depends on whether the data is parametric (something which can be measured, usually normally distributed) or non-parametric.
Parametric tests:
Non-parametric tests:
Paired data:
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