Data handling — overall % change must be weighted by group sizes, not a mean of percentages
“Candidates selecting option D had assumed that the overall percentage reduction… was the mean of the percentage reductions in the individual areas. This would only be true if the numbers of fish in each area… were the same. [Option B] assumed any change could be attributed to global warming.”
Set each area’s starting fish count and its percentage drop. Compare the (wrong) simple mean of percentages with the (correct) overall percentage from the totals.
Make the areas equal and the two figures coincide; make them unequal and the simple mean misleads. Always weight by group size.
Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Biology 8876 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.
Made by lookang, using Claude Fable 5. More resources: iwant2study.org