What do histograms do

The histogram is a popular graphing tool. It is used to summarize discrete or continuous data that are measured on an interval scale.

It is often used to illustrate the major features of the distribution of the data in a convenient form. It is also useful when dealing with large data sets greater than observations.

It can help detect any unusual observations outliers or any gaps in the data. A histogram divides up the range of possible values in a data set into classes or groups. For each group, a rectangle is constructed with a base length equal to the range of values in that specific group and a length equal to the number of observations falling into that group. A histogram has an appearance similar to a vertical bar chart, but there are no gaps between the bars. Generally, a histogram will have bars of equal width.

Chart 5. Note that other distributions look similar to the normal distribution. Statistical calculations must be used to prove a normal distribution. It's important to note that "normal" refers to the typical distribution for a particular process. For example, many processes have a natural limit on one side and will produce skewed distributions. The skewed distribution is asymmetrical because a natural limit prevents outcomes on one side.

For example, a distribution of analyses of a very pure product would be skewed, because the product cannot be more than percent pure. Other examples of natural limits are holes that cannot be smaller than the diameter of the drill bit or call-handling times that cannot be less than zero.

These distributions are called right- or left-skewed according to the direction of the tail. The bimodal distribution looks like the back of a two-humped camel. The outcomes of two processes with different distributions are combined in one set of data. For example, a distribution of production data from a two-shift operation might be bimodal, if each shift produces a different distribution of results. Stratification often reveals this problem. You can see for example that there are 30 trees from cm to just below cm tall.

Notice that the horizontal axis is continuous like a number line :. Histograms are a great way to show results of continuous data , such as:. A Frequency Histogram is a special graph that uses vertical columns to show frequencies how many times each score occurs :.