Which of the following is a disadvantage of matched pairs design?

A matched pairs design involves pairing participants based on certain characteristics and then assigning them to different conditions within an experiment. One of the significant disadvantages of this design is the need for a large pool of participants. This requirement arises because to effectively match individuals according to specific traits, researchers often need a broad sample size to ensure that pairs can be accurately formed across various conditions. If the sample size is limited, it can result in difficulties in finding suitable matches, potentially leading to a reduced validity of the findings due to unbalanced group characteristics. This substantial need can pose logistical challenges and may complicate recruitment efforts for researchers.

In contrast, other options tend to represent advantages or unrelated issues in the context of matched pairs design. For instance, decreased reliability of results and increased participant motivation are typically not associated problems of this design; a well-implemented matched pairs design can often produce reliable results. Similarly, while demand characteristics can be a concern in experimental research generally, they are not unique to or exacerbated by the matched pairs design itself. Hence, the need for a large pool of participants stands out as a notable disadvantage specific to this method.