Four Types of Scales are used to interpret the world in numbers.

Note

The scales are hierarchically arranged from least information provided (nominal) to most information provided (ratio). Any scale can be degraded to a lower scale, e.g., interval data can be treated as ordinal.

For the USMLE, concentrate on identifying nominal and interval scales.

Parametric Tests (something which can be measured, or normally distributed)

1. Student's t-test - paired or unpaired
2. Pearson's product- moment coefficient - correlation

Non-Parametric Tests

• Mann-Whitney => unpaired data
• Wilcoxon matched-pairs => compares two sets of observations on a single sample
• Chi-square
• Spearman, Kendall rank - Correlation
• McNemar's test is used on nominal data to determine whether the row and column marginal frequencies are equal

Paired data refers to data obtained from a single group of patients,

e.g. Measurement before and after an intervention.

Unpaired data comes from two different groups of patients,

e.g. Comparing response to different interventions in two groups

For interval data => use Correlation

For nominal/categorical data => use Chi-square