AP Psychology Practice Test

The correlation coefficient measures how well two variables vary together and predict each other, which directly aligns with the nature of correlation itself. This statistical tool quantifies the strength and direction of the relationship between two variables. A positive correlation indicates that as one variable increases, the other tends to increase as well, while a negative correlation indicates that as one variable increases, the other tends to decrease. This measure is especially valuable in determining the degree to which changes in one variable might correspond to changes in another, providing insights into potential predictive relationships.

In contrast, the other options focus on different concepts unrelated to the specific function of the correlation coefficient. For instance, naturalistic observation refers to observing subjects in their natural environment without manipulation, whereas the effectiveness of an experiment relates to how well it can test a hypothesis. Lastly, random selection pertains to techniques used in survey populations to ensure that every individual has an equal chance of selection, which is important for generalizability but doesn't specifically relate to the measurement of correlation.