Why is it wrong for a researcher to assign the first subjects to show up for an experiment to the experimental group and the second group of subjects to arrive to the control group?

Answer 1

Because then the grouping is (technically) not random. In order for an experiment to yield proper results, the grouping must be random. If the researcher assigns subjects to groups in the order in which they arrive, then the randomness of the experiment is not present.

cw: There are experiments on experiments that show that the first half of subjects are different from the second half of subjects. Why is another question. So you should randomize the allotment of subjects to treatments (groups).

Answer 2

Assigning subjects based on arrival time can introduce bias into the study, as there could be systematic differences between subjects arriving early and those arriving late. This could confound the results of the experiment and make it difficult to determine the true effect of the treatment. It is essential to randomly assign subjects to experimental and control groups to ensure the groups are comparable at the start of the study.