We also need to decide what to measure . In this case, it would be whether the person sneezes or not. But we might also want to record if they start to sneeze but stop short, as well as recording how often they sneeze. But now comes the hard part 3. The hard part about designing experiments is this step.
Some examples follow.
This has two confounding factors. Finalizing the experimental design. For example, you might not want to include anyone in the experiment that routinely sunbathes, alters their hair color or wears contact lenses, etc. Note, that it is important to know as much of this information about the confounding variables as possible, for each subject. So part of the designing the experiment is to think of the confounding variables and then ask this information of the experimental subjects.