Reliability vs validity

Explain the difference between reliability and validity when evaluating fieldwork data, using a fieldwork example. [6 marks]

Mark scheme (AO3, 6 marks — 2 per definition + 2 for example):

• 1–2 marks: reliability — the extent to which repeating the method would produce the same results; depends on sample size, consistent technique.
• 1–2 marks: validity — whether the data actually measures what was intended and answers the enquiry question.
• 1–2 marks: a worked example showing both — e.g. a pedestrian count taken once on a bank holiday may be reliable (repeatable on similar days) but invalid for assessing typical weekday footfall.

Grade-9 exemplar: Reliability concerns repeatability — counting pedestrians for 10 minutes at three sites three times yields consistent figures, so the data is reliable. Validity concerns whether the data answer the question — if collected on a bank holiday during an event, the data are invalid as a measure of typical footfall, even if reliable. Grade-9 evaluation distinguishes these clearly and proposes triangulation across multiple days as an improvement.