When to use it
Use it when groups contribute different volumes or exposures.
An average of percentages need not equal the overall percentage; percentage points differ from percentage change.
Apply it
Use nonnegative weights with a positive sum. Percentage change is (new − old)/old × 100% when old is nonzero; a change between percentages is measured in percentage points.
- Identify the numerator, denominator and unit of every input.
- Choose weights that reflect the population or decision.
- Check whether aggregation hides important subgroup differences.
An example in practice
Combine completion counts across teams using their eligible workloads, rather than taking an unweighted average of team completion rates.
Source & credit
Standard mathematical statistics; the handbook is an authoritative technical reference, not a claim of sole origin.
NIST/SEMATECH: statistical methods
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