Final answer:
The correct match is C) Placebo effect - Participants in the treatment group expect improvement, leading to actual improvement. The placebo effect is important in research and can affect the study results, which is why control groups and double-blind designs are crucial to maintaining the validity of an experiment.
Step-by-step explanation:
The threat to the validity of outcome research correctly matched with its definition is C) Placebo effect - Participants in the treatment group expect improvement, leading to actual improvement. The placebo effect stems from the participant's expectation to feel better simply because they believe they are receiving an actual treatment, which can cause them to report an improvement in their condition, even though they have received an inert substance. This makes it essential to include control groups in research to compare the effects of the active treatment against the placebo.
Selection bias occurs when participants' characteristics systematically differ between groups, compromising the study's validity by making the groups incomparable. Regression toward the mean explains the phenomenon where extreme scores or behaviors tend to become less extreme over time. Attrition pertains to the issue of participants dropping out of the study, potentially leading to a biased sample because those who remain may not be representative of the original group.
A randomized experiment with a double-blind design, where participants are selected through a random sampling process, and some receive a placebo, helps minimize these biases. With a properly designed study, the researcher can attribute differences in outcomes to the influence of the explanatory variable rather than these threats to validity.