Set out by John Stuart Mill (A System of Logic, 1843).
Compares cases with and without an effect using five set patterns, agreement, difference, the two combined, residues, and concomitant variation, to narrow down its likely cause.
When to reach for it
When cases can be compared but not randomized, as in an outage, an incident review, or a puzzle about why one customer segment behaves differently.
How it runs
- For each case, record the features present and whether the effect occurred.
- Pick the method the data supports.
- Apply it to isolate candidate causes.
- Hold the result as a lead to test; confounders can mislead.
- Confirm with an intervention or a discriminating observation.
Where it sits in SOLVE eX
Go to the source
- A System of Logic, Ratiocinative and Inductive, John W. Parker, London; Book III, chapter 8, 1843