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Situations in which the matched-pair design is (or might be) better and those in which the stratified design is (or might be) better (shading indicates the situations prevailing in our study) |
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| Matched pairs better |
Stratified better |
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| Large variation between pairs with respect to baseline risk |
Within-stratum variation small compared to between-stratum variation |
| High matching correlation |
Small matching correlation within strata |
| No individual level analysis desired |
Analysis at individual level desired (2) (interactions of interventions with age, gender, medical history) |
| Homogeneity of effect can be assumed across pairs |
Heterogeneity of effect across strata possible |
| No drop-outs expected |
Individual hospitals may drop out |
| Medium number of clusters (20 to 40, so as to have 10–20 well-matched pairs) |
Large number of clusters (perhaps >30 or >40 depending on the number of strata) |
| Calculation of ICC(1) needs special assumptions |
Calculation of ICC straightforward |
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(1) ICC – intracluster correlation coefficient (2) Only as secondary analysis in our study | |
Gülmezoglu et al. BMC Medical Research Methodology 2004 4:2 doi:10.1186/1471-2288-4-2 |
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