Definition
Casework
Casework solves or proves something by splitting all possibilities into separate, complete cases.
What it means
Break a large problem into a few clear “if this, then…” parts. Every possibility must belong to a case, and no possibility should be counted twice. When each case has been solved, put the results together.
Remember it
Cover every box once, then combine what the boxes tell you.
Example
To count two-digit numbers made from 1, 2, and 3 without repeating a digit, split by the tens digit. Starting with 1 gives 12 and 13; with 2 gives 21 and 23; with 3 gives 31 and 32. The three cases cover all 6 numbers once.
Watch for
Cases may overlap only if the overlap is handled deliberately; otherwise results can be double-counted.
Common mix-up
Trying a few examples is not casework unless the cases cover every possibility.
Exact meaning
Casework is a method that partitions all possible situations into exhaustive, non-overlapping cases, handles each case separately, and then combines the conclusions.