Statistics in Psychology

A branch of mathematics devoted to the collection, compilation, display, and interpretation of numerical data.

Psychologists rely heavily on statistics to help assess the meaning of the measurements they make. Sometimes the measurements involve individuals who complete psychological tests; at other times, the measurements involve statistics that describe general properties of groups of people or animals.

In psychological testing, the psychologist may interpret test results in light of norms, or the typical results, provided from previous testing. In research, psychologists use two kinds of statistics, descriptive and inferential. Descriptive statistics simply give a general picture of the scores in a given group. They include the measures of central tendency and the measures of variability. Central tendency involves different kinds of averages: the mean, median, and mode. Variability involves the standard deviation, which indicates how far scores in a group are likely to be from the average.

Inferential statistics are used to help psychologists draw inferences, or conclusions, from the data obtained from their research. The most common statistical tests include the student's T-test and the Analysis of Variance (or F-test); these statistics help the psychologist assess whether the differences in averages across groups are due to the effects of an independent variable. Another widely used inferential statistic is the correlation coefficient, which describes the strength of the relationship between two variables. For example, there is a positive correlation between a student's score on the Scholastic Assessment Test (SAT) and his/her grades in the first year of college. Correlations involve patterns that exist in groups; individuals within those groups may not perform in the manner the correlation predicts that they will, but if large numbers of students are tested, general trends may be detected.