Tension is rising in the Harford household as exams approach and we try to persuade Miss Harford Sr to relax, and Miss Harford Jr to be slightly less relaxed. I'm sure many readers have vivid memories of the exam room, recent or otherwise.

But here's a question about exam technique that suggests a much wider lesson. In a multiple-choice test, you sometimes write down an answer and then have second thoughts. Is it wise to stay with your first instincts, or better to switch?