Histogram Bob Ollerton

Histogram

• Bob Ollerton

• Sector Enterprise Quality – Quality and Mission Assurance

• Northrop Grumman Corporation

• Integrated Systems

Why use a Histogram

• To summarize data from a process that has been collected over a period of time, and graphically present its frequency distribution in bar form.

What Does a Histogram Do?

• Displays large amounts of data that are difficult to interpret in tabular form

• Shows the relative frequency of occurrence of the various data values

• Reveals the centering, variation, and shape of the data

• Illustrates quickly the underlying distribution of the data

• Provides useful information for predicting future performance of the process

• Helps to indicate if there has been a change in the process

• Helps answer the question “Is the process capable of meeting my customer requirements?”

How do I do it?

• Decide on the process measure

• The data should be variable data, i.e., measured on a continuous scale. For example: temperature, time, dimensions, weight, speed.

• Gather data

• Collect at least 50 to 100 data points if you plan on looking for patterns and calculating the distribution’s centering (mean), spread (variation), and shape. You might also consider collecting data for a specified period of time: hour, shift, day, week, etc.
• Use historical data to find patterns or to use as a baseline measure of past performance.

How do I do it? (cont’d)

• Prepare a frequency table from the data

• a. Count the number of data points, n, in the sample

How do I do it? (cont’d)

• Tip: The number of intervals can influence the pattern of the sample. Too few intervals will produce a tight, high pattern. Too many intervals will produce a spread out, flat pattern.

• Determine the class width, H.
• The formula for this is:
• H = R = 1.7 = 0.17
• k 10
• Round your number to the nearest value with the same decimal numbers as the original sample. In our example, we would round up to 0.20. It is useful to have intervals defined to one more decimal place than the data collected.
• Determine the class boundaries, or end points.
• Use the smallest individual measurement in the sample, or round to the next appropriate lowest round number. This will be the lower end point for the first class interval. In our example this would be 9.0.

How do I do it? (cont’d)

• Add the class width, H, to the lower end point. This will be the lower end point for the next class interval. For our example:
• 9.0 + H = 9.0 + 0.20 = 9.20
• Thus, the first class interval would be 9.00 and everything up to, but not including 9.20, that is, 9.00 through 9.19. The second class interval would begin at 9.20 and everything up to, but not including 9.40.
• Tip: Each class interval would be mutually exclusive, that is, every data point will fit into one, and only one class interval.
• Consecutively add the class width to the lowest class boundary until the K class intervals and/or the range of all the numbers are obtained.

How do I do it? (cont’d)

• Construct the frequency table based on the values you computed in item “e”.
• A frequency table based on the data from our example is show below.

How do I do it? (cont’d)

• Draw a Histogram from the frequency table

• On the vertical line, (y axis), draw the frequency (count) scale to cover class interval with the highest frequency count.
• On the horizontal line, (x axis), draw the scale related to the variable you are measuring.
• For each class interval, draw a bar with the height equal to the frequency tally of that class.

How do I do it? (cont’d)

• Interpret the Histogram

• Centering. Where is the distribution centered?
• Is the process running too high? Too low?

How do I do it? (cont’d)

• b. Variation. What is the variation or spread of the data? Is it too variable?

How do I do it? (cont’d)

• c. Shape. What is the shape? Does it look like a normal, bell-shaped distribution? Is it positively or negatively skewed, that is, more data values to the left or to the right? Are there twin (bi-modal) or multiple peaks?

How do I do it? (cont’d)

• d. Process Capability. Compare the results of your Histogram to your customer requirements or specifications. Is your process capable of meeting the requirements, i.e., is the Histogram centered on the target and within the specification limits?

How do I do it? (cont’d)

• Tip: Get suspicious of the accuracy of the data if the Histogram suddenly stops at one point (such as a specification limit) without some previous decline in the data. It could indicate that defective product is being sorted out and is not included in the sample.

• Tip: The Histogram is related to the Control Chart. Like a Control Chart, a normally distributed Histogram will have almost all its values within +/-3 standard deviations of the mean. See Process Capability for an illustration of this.

Histogram