Frequency Distributions

There are several ways in which we can depict a frequency distribution of a set of raw scores. Frequency tables are comprised of two columns, one with all possible values and one with a count of how often each value occurs. Grouped frequency tables allow us to work with more complicated data. Instead of containing values, the first column consists of intervals. Histograms display bars of different heights indicating the frequency of each value (or interval) that the variable can take on. Frequency polygons show frequencies with dots at different heights depicting the frequency of each value (or interval) that the variable can take on. The dots in a frequency polygon are connected to form the shape of the data.

Shapes of Distributions

The normal distribution is a specific distribution that is unimodal, symmetric, and bell- shaped. Data can also display skewness. A distribution that is positively skewed has a tail in a positive direction (to the right), indicating more extreme scores above the center. It sometimes results from a floor effect, in which scores are constrained and cannot be below a certain number. A distribution that is negatively skewed has a tail in a negative direction (to the left), indicating more extreme scores below the center. It sometimes results from a ceiling effect, in which scores are constrained and cannot be above a certain number.