### (3) Histograms

Although the overall distribution pattern can be deduced from the data points in a frequency distribution table, the distribution can also be represented as a column graph, known as a histogram (see Figure “A Histogram”). The histogram is employed to reveal mean values and variation patterns, the histogram plays an important role as a process analysis technique for tasks such as checking for defectives by comparing product quality characteristics against standard values.

#### 1 Frequency Distribution Tables

In a frequency distribution table, quality characteristic data are sorted into several regularly-spaced classes, arranged in size order, to find how many values each class has. The frequency of a class is the number of values appearing in that class (or the number of times the same value appears, if each data value is taken as a class), and a frequency distribution table is a tabulation of the number of data points in each class.
A good indication of the overall distribution pattern can usually be found by gathering a hundred or more data points and drawing up a frequency distribution table by distributing them between 10 to 20 classes.

#### 2 Normal Distribution

The normal distribution is the most common distribution pattern for a variable. It produces a bell-shaped distribution curve that is laterally symmetrical about a central line (see Figure “Normal Distribution”).