Odd one out: the intended category
Any set of five items can be split in several defensible ways. The task is to find the split the item writer built in.
Look for the strong property first
Numbers: check squares, primes, multiples. Words: check origin, category and function. The rule that cleanly covers four and excludes one is the intended rule.
Worked example
Ruby, emerald, sapphire, topaz are minerals. A pearl is produced by a living organism. That is one property splitting four against one; colour is not.
When two rules compete
Prefer the one that needs no exception. If your rule requires a caveat to hold, it is probably not the one being tested.
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Questions people ask
Is there always one right answer?
Well-written items have exactly one rule that separates four from one without exception.
How do I speed these up?
Name the shared property of the majority first, then look for the item that fails it.