Synthesizing Evidence
When many studies address the same question, two methods synthesize the evidence. A systematic review comprehensively identifies, evaluates, and summarizes all available research using an explicit process to avoid cherry-picking.
A meta-analysis goes a step further, statistically combining numerical effect sizes into one precise overall estimate, pooling samples to increase statistical power.
| Method | Core Function |
|---|---|
| Systematic review | Summarizes existing research |
| Meta-analysis | Combines numerical results |
Both protect against bias and turn scattered findings into clear conclusions.
Tracking Change Over Time
Studies tracking change use two contrasting designs. A longitudinal design follows the same individuals over time, directly measuring real change within each person.
A cross-sectional design compares different age groups at one point in time. It is faster and cheaper, but vulnerable to cohort effects: generational differences in life experience that masquerade as true age changes.
| Design | Tracks | Strength | Weakness |
|---|---|---|---|
| Longitudinal | Same group | Measures real change | Slow, costly, attrition |
| Cross-sectional | Different groups | Fast, cheap | Cohort effects |
Pitfall: Treating a cross-sectional age difference as proof of aging. It often just reflects different historical eras.