In the potato practical, cylinders may start with slightly different masses. To compare them fairly we use the percentage change in mass rather than the raw change.
Percentage change in mass = (final mass - initial mass) ÷ initial mass × 100. A positive answer is a percentage gain. A negative answer means the potato has lost mass because water left its cells by osmosis, and we can call it a percentage loss.
Example 1: a cylinder goes from an initial mass of 2.50 g to a final mass of 2.80 g. The change is 0.30 g. 0.30 ÷ 2.50 = 0.12, and 0.12 × 100 gives a gain of 12%.
Example 2: a cylinder falls from 4.0 g to 3.0 g. The mass lost is 1.0 g. 1.0 ÷ 4.0 = 0.25, so the percentage loss is 25%. Always divide by the initial mass, not the final mass.