Which sample type is most likely to be affected by self-selection bias?

A volunteer sample is most likely to be affected by self-selection bias because individuals who choose to participate in a study often have specific characteristics or motivations that differentiate them from those who do not choose to participate. This means that the sample may not be representative of the broader population, as it tends to attract individuals who are particularly interested or have a strong opinion about the topic being studied.

In contrast, an opportunity sample involves selecting participants who are easily accessible, and while this can also introduce some bias, it is generally not as influenced by individual motivations for participation. A random sample aims to minimize bias by giving every member of the population an equal chance of being selected, thereby enhancing representativeness. A stratified sample is designed to reflect the characteristics of the population by ensuring specific subgroups are adequately represented. Thus, volunteer samples are particularly vulnerable to self-selection bias due to the inherent nature of who opts in to participate.