Histogram as a Component of Seven Basic Quality Tool

Histogram is a bar chart to show frequencies of causes of problems to understand preventive or corrective action.
PMBOK(r) Guide fifth Edition defines Histogram as:

“A special form of bar chart used to describe the central tendency, dispersion, and shape of a statistical distribution”

A histogram is a way to represent tabulated frequencies shown as adjacent rectangles.

Development of Histogram:

Check sheet is used as an input to develop histogram, Please refer to the Blog Check Sheet as a Component of Seven Basic Quality Tool 
To get more details about check sheet;
As a reference, I am using the same example as used in check sheet:
An IT test team member is evaluating work products to detect problems from the specifications. Team may choose to categorise data about quality problems in following categories:
Categories suggested by Roger S. Pressman:

1. Incomplete or erroneous specification (IES)
2. Misinterpretation of customer communication (MCC)
3. Intentional deviation from specifications (IDS)
4. Violations of programming standards. (VPS)
5. Error in data representations (EDR)
6. Inconsistent component interface (ICI)
7. Error in design logic (EDL)
8. Incomplete or erroneous testing (IET)
9. Inaccurate or inconsistent documentation (IID)
10. Error in programming language translation of design (PLT)
11. Ambiguous or inconsistent human/computer interface (HCI)
12. Miscellaneous (MIS)

While examination the work product, test team member assesses the defects and enter frequencies in their respective category of causes like:
Check Sheet suggested by roger s. pressman in software engineering a practitioner’s approach

Error Causes	Total		Serious		Moderate		Minor
Error	Frequency	%	Frequency	%	Frequency	%	Frequency	%
IES	205	22%	34	27%	68	18%	103	24%
MCC	156	17%	12	9%	68	18%	76	17%
IDS	48	5%	1	1%	24	6%	23	5%
VPS	25	3%	0	0%	15	4%	10	2%
EDR	130	14%	26	20%	68	18%	36	8%
ICI	58	6%	9	7%	18	5%	31	7%
EDL	45	5%	14	11%	12	3%	19	4%
IET	95	10%	12	9%	35	9%	48	11%
IID	36	4%	2	2%	20	5%	14	3%
PLT	60	6%	15	12%	19	5%	26	6%
HCI	28	3%	3	2%	17	4%	8	2%
MIS	56	6%	0	0%	15	4%	41	9%
Total	942	100%	128	100%	379	100%	435	100%

As a further explanation, first two columns are used to develop Pareto chart, as shown below:

Error  Causes	Frequency
IES	205
MCC	156
IDS	48
VPS	25
EDR	130
ICI	58
EDL	45
IET	95
IID	36
PLT	60
HCI	28
MIS	56
Total	942

Now using excel we can develop Histogram, as mentioned below:

Histogram is showing using bar chart that most of the frequencies of causes are coming from IES. MCC and EDR and we need to take corrective action to solve the source of problems.

Usage of histogram:

I am describing usage of Histogram in context of “Plan Quality” and “Control Quality”
When Histogram is used in “Plan Quality”, serves as a preventive approach to improve processes where historical data is used to identify categories of causes effecting most. Processes are selected to improve, for example due to higher frequencies in IES, MCC and EDR we may select improvements in “Collect Requirement”, “Define Scope” processes.
Histogram is used in “Control Quality” to identify causes of poor performance in process and work products.
Difference between Pareto Chart and Histogram:

Histogram	Pareto Chart
Histogram is a kind of bar chart showing a distribution of variables or causes of problems.	A Pareto chart is a specific type of histogram that represents causes of problems by their overall influence. This is an effective tool to prioritise corrective action as errors with greatest impact are displayed in descending order of frequency.
A histogram represents cause of a problem as a column and the frequency of each cause of problem as the height of the column.	In addition an arc representing the cumulative percentage of frequencies of causes is also included.

In short, A histogram is a bar chart that show the frequency of a cause of a problem occurring using the height of the bar as an indicator.
I hope this blog has sufficiently answered your all queries related to Histogram. Good Luck with your PMP® Certification Exam.
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