National EMSC Data Analysis Resource Center
National EMSC Data Analysis Resource Center
| Groups | Purpose | Conditions or Assumptions | Test | Descriptive Measures (also used to calculate CIs) |
Compare mean to historical or hypothetical (known) mean |
Standard deviation known or N large and approximate normality |
One-sample z-test |
Mean, std deviation, difference from historical mean |
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Compare mean to historical or hypothetical (known) mean |
Standard deviation unknown; approximate normality |
One-sample t-test |
Mean, std deviation, difference from historical mean |
Compare means |
Approximate normality |
Two-sample t-test |
Means, std deviations, pooled std deviation, difference in means |
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|
Compare medians (Pr (X < Y)) |
Use for highly non-normal outcomes |
Wilcoxon rank-sum test Mann-Whitney U test |
Medians, quartiles, difference between medians |
Compare means (any or overall difference) |
Approximate normality |
Analysis of variance (ANOVA) |
Means, std deviations, pairwise differences in means |
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|
Compare medians (outcome distributions) |
Use for highly non-normal outcomes |
Kruskal-Wallis test |
Medians, quartiles, pairwise differences in medians |
Evaluate whether mean of differences nonzero |
Approximate normality |
Paired t-test |
Mean of differences, std deviation of differences |
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Evaluate whether median of differences nonzero |
Use for highly non-normal outcomes |
Wilcoxon sign-rank test |
Median of differences, quartiles of differences |
Evaluate whether mean of differences nonzero |
Approximate normality |
Repeated measures ANOVA |
Mean of differences, std deviation of differences |
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|
Evaluate whether median of differences nonzero |
Use for highly non-normal outcomes |
Friedman test |
Median of differences, quartiles of differences |
rev. 08-Sep-2011
