Mrs Hughes's Observation — Detect the Flaw

Mrs Hughes sees seven glasses-wearers among her ten short-sighted students and immediately concludes the rest must be wearing contact lenses. It sounds logical — but she has forgotten a third option that changes everything. This is one of the most common reasoning traps in OC TS: assuming only two possibilities exist when at least one more is hiding in plain sight.

The "missing alternative" error appears frequently across OC TS detecting-errors questions, dressed in many different real-world situations — eyesight, travel, food preferences, behaviour. The pattern is always the same: a person sees two options (A or B) and concludes the absence of A proves B, while a perfectly valid option C was never considered.

The examiner wants to confirm that you can identify when a conclusion is reached by eliminating only one alternative instead of all alternatives. The key question to ask is: "Has the speaker assumed these are the ONLY two options?" If yes, that is the mistake — and the correct answer will name the overlooked alternative.

A person observes something (e.g. seven out of ten students wearing glasses), subtracts it from a total, and concludes the difference must be explained by one specific thing (contact lenses). The correct answer identifies the unacknowledged third (or fourth) option that could also explain the difference.