3P.3—Experimental design and method evaluation

A fair experiment changes one independent variable, measures a defined dependent variable and controls other factors that could alter the result. The method must make the comparison reproducible.

Specify the sample and range, keep volume, temperature, time, concentration and apparatus consistent where relevant, repeat measurements and include a control. Choose a measurement that responds directly to the process being tested.

To compare plant extracts, use equal discs, equal soaking time, the same bacterial lawn and a solvent-only control; measure clear-zone diameter or area after the same incubation.

A control is not merely “another group”; it must isolate the effect of the independent variable. Repeats reduce random variation but cannot repair systematic bias or a poorly defined endpoint.