StatTools : Sample Size for Matched Pair Comparison of Two Proportions
Explanations and Tables

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Sample Size Introduction and Explanation Page
Unpaired Comparison of Two Proportions Program Page
Unpaired Proportions Explanations Page
Sample Size for Matched Paired Controlled Studies Program Page

Introduction Sample Size Tables References
Introduction Example
Matched pair control studies is an important epidemiological tool that allows associations between effects and its suspected causes to be tested. A typical example was in the search for a cause of newborn limb defects, when Thalidomide was suspected. For each case of limb defect, a number of matched babies with no limb defect was selected, and the mothers were asked for exposure to Thalidomide in early pregnancy, and it was found that mothers of babies with limb defects were much more likely to have been exposed to Thalidomide.

The matched pair controlled study allows each case with an indicated outcome to be matched by more than 1 case of control, thus increasing the statistical power and alleviates the problems associated with uncommon conditions. It also allows a retrospective examination of causes, using outcome to classify the groups.

The statistical method is usually the Odds Ratio, as the retrospective nature and the manner of case selection does not allow risks to be computed.

Sample size calculation during planning, and power analysis at the conclusion of studies are therefore important to ensure a robust and sensitive conclusion.

The model links outcomes backwards to causation. A case with an indexed outcome (e.g. fetal malformation) is matched with one or more normal cases (control), so that as many possible confounding variable as possible are matched (age, social class, and so on). The incidence of exposure to a specified suspected cause (e.g. exposure to a drug) in the two groups are then compared

The study unit is the matched unit, an index case and one or more controlled cases. Sample size and power calculations are based on the number of units.

As matched pair studies are focussed on identifying a cause of an outcome, the one tail model is usually used. The calculations and tables from StatTools are therefore all one tail models.