2.1.1 Histogram

The word histogram comes from the Greek histos, meaning pole or mast, and gram, which means a chart or graph. Hence, the direct definition of Histogram is Pole Chart. A histogram is used to display the distribution of data values along the real number line. It is a traditional way of displaying the shape of data and competes with the probability plot as a method of assessing normality.

Histogram is a graph of the frequency distribution in which the vertical axis represents the count (frequency) and the horizontal axis represents the possible range of the data values. It is constructed from a frequency distribution, where choices on the number of classes and class width have been made. These choices can drastically affect the shape of the histogram With GRAPHID, however, the number of classes can be increased or decreased interactively to visualize the effect of changing the class intervals. The ideal shape to look for in the case of normality is a bell-shaped symmetrical distribution. If the histogram does not look like a (nearly) bell-shaped distribution, one can standardize the variables and plot again the histograms to explore whether standardization yields the expected results. Standardization can be done within the module itself.

GRAPHID offers the possibility of viewing histograms of a number of variables, one after another. The number of class intervals can be increased to approximate a histogram to a probability plot.