bar chart

A graphic means of displaying a frequency distribution of nominal, ordinal, or grouped continuous data. Blocks proportional in width to the size of the categories are raised along the horizontal axis. The height of each block is then adjusted so that its area is proportional to the relative frequency of its category. The diagram thus appears as a series of bars of varying height, as in the hypothetical example in Figure 1, which shows the number of births in a county hospital for each day of the week. Conventionally, the frequency is shown at the top of each bar, and only values which actually occur are represented. (For example, if there had been no births on Monday, no space would have been left for that day.)

FIGURE 1: A bar chart showing frequency of births by day of week

Other graphic methods of displaying data include the histogram and frequency polygon. The former is a graph showing interval-level or ratio-level data, consisting of contiguous lines or bars of a height corresponding to the number of observations in the interval, and should only be used to display variables when there is an underlying order to the values. Conventionally, intervals that have no observations are included in the histogram, although no lines or bars are printed. Figure 2 is a hypothetical example showing the monthly spending of a group of teenagers attending a summer camp. (The numbers below the bars indicate the middle value or mid-point of each interval.) A variation on this technique is the ‘frequency polygon’, in which the bars are replaced by dots placed above the mid-point of each class interval, and the dots then joined by straight lines.

These and other visual means of presenting data (such as the pie-chart ) are normally found where researchers wish to convey the basic information contained in a frequency distribution at a glance or in a simplified form. For this reason they are widely used in media such as magazines and newspapers. See also measurement.

FIGURE 2: A histogram of monthly expenditure