Measures of Central Tendency

Measures of central tendency are indices that represent the typical or average score among a distribution of scores. Measures of central tendency can describe a set of numerical data with a single number. The three most frequently used measures of central tendency are the mean, median, and mode. The choice usually depends on two factors: the type of measurement scale used and the purpose of the research. There are four measurement scales: nominal, ordinal, interval, and ratio.

Nominal data classify persons or objects into two or more categories: sex (male or female), type of school (public or private), IQ (high, average, low), political party (Democrat or Republican), personality type (dominant or passive), race/ethnicity (African American, Asian, Hispanic, White). Ordinal data not only classify persons or objects but also rank them in terms of degree to which they possess a characteristic of interest. In other words, ordinal data puts participants in order from highest to lowest. For example, 15 doctoral cohort members might be ranked from 1 to 15 with respect to height. Percentile ranks are ordinal data. Most standardized tests, like the GRE, provide a raw score, as well as a percentile rank from 100 to 0. Interval data have all of the characteristics of nominal and ordinal data, but in addition, they are based on predetermined equal intervals. Most tests used in social science research, such as achievement tests, aptitude tests, and intelligence tests, represent interval data. Ratio data are derived from scales that have an absolute zero and so enable relative comparisons to be made, such as length of school day, class size, age, speed, and dollars.