A histogram is a graphical representation of data as a frequency distribution. This tool is valuable in evaluating both attribute (pass/fail) and variable (measurement) data. Histograms offer a quick look at the data at a single point in time; they do not display variance or trends over time. A histogram displays how the cu- mulative data look today. It is useful in understanding the relative frequencies (percentages) or frequency (numbers) of the data and how those data are distributed. Figure 20-15 illustrates a histogram of the frequency of defects in a manufacturing process.