Final answer:
Self-selection restricts test score range by narrowing participant characteristics, leading to a non-representative and more homogeneous sample. Random sampling ensures everyone has an equal chance to participate, thereby enhancing representation and study validity.
Step-by-step explanation:
Self-selection during participant recruitment restricts the range of test scores by b) narrowing the range of participant characteristics. This occurs because individuals who choose to participate may share certain attributes or behaviours that are not representative of the entire population. As a result, the sample could be biased, and the variability of the test scores might be reduced since the participants are more homogeneous than the population at large.
Regarding random sampling, d) Everyone has the same chance of being part of the study is what makes it an effective method to select participants. This enhances the likelihood that the sample will represent the population, contributing to the reliability and validity of the results. Selection measures, like random sampling, aim to avoid issues like self-selection bias to improve the accuracy of the findings.