Histogram

Histograms are another method of looking at variation by looking at the frequency the data is distributed. They are bar charts that display this data distribution, using the number of times the data points appear in each bar. In this manner, they show how consistently a process meets its goals. For instance, if your goal is to achieve a ten minute ECG interpretation from the time the patient enters the ED, you would gather all your times for a given period, categorize them into sections or into individual times depending on the size of the sample, and plot each time within the category.

Histograms can tell you what your distribution or variation looks like and whether or not your data contain any outliers. The example below looks at 22 door-to-ECG times.

In this example, 22 door-to-ECG times were measured and categorized into blocks of time.

• 6 times the door-to-ECG time was 11 minutes - the "mean"
• 4 times the door-to-ECG time was 10 minutes and 4 times the door-to-ECG time was 12 minutes
• 3 times the door-to-ECG time was 9 minutes and 3 times the door-to-ECG time was 13 minutes
• 1 time the door-to-ECG time was 8 minutes and 1 time the door-to-ECG time was 14 minutes

The process of obtaining the ECG appears to be in control with little variation. We know this because, with a controlled process you would see that most times are close to the average with fewer times occurring the further you get from the average.

Now there is an opportunity to improve the process becaue it appears to be well controlled, or done in a consistent manner each time.

*Although 22 ECG times were used for the above example, larger numbers are typically more helpful in determining whether or not variation exists in a process.