Tool 5 - Pareto chart

A Pareto chart is a form of a Bar chart which the special characteristic that the data is sorted in descending order before being used in the bar chart. As such the Pareto chart provides an immediate visual trend of the data set.

Because the Pareto chart pre-arranges the raw data, it is efficient when identifying most significant problems or causes, especially when there are several. Based on it's inherent property of representing frequency of occurrence, the Pareto chart is an effective tool to analyze frquency distribution for various categories.

To build a Pareto chart, it is important to correctly select the categories to be displayed and also the appropriate scale relative to the data collected. The Check sheet can be used to gather data. Once the Pareto chart is generated, the categories starting from the left are the obvious choices for further analysis.

Tool 4 - Histogram

A histogram is one of the most commonly used tool to display frequency distribution of events. It shows how often each different value occurs within a set of observations. 

Although quite often used interchangably with a Bar chart, there are major differences; a histogram represents density distribution of quantitative variables, whereas the Bar chart repsents frequency distribution of categorical variables. This means in a histogram it is the area of the bar that denotes the value, whereas in a Bar chart it is the height.

A histogram is most efficient when data under analysis is numerical. Generating a histogram involves determining the number of classes (bars in the graph) to be used, the range of data values and the labels for the bar edges.

As a histogram can display tendency of the process/product measurements, it is an effective tool to study whether requisite requirements can be met.

Tool 3 - Control chart

A control chart is a graph that represents the behaviour of a process over time. Also called a Shewhart chart, this statistical process control tool is used to indicate whether a process is functioning in control. The Control chart can provide critical information on operational parameters of a process, and so is sometimes called the process-behaviour chart.

The control chart plots measurements of a specific process quality characteristic (Y-axis) against time (X-axis). Predetermined measurement values are used to set the Upper (UCL) and Lower control limits (LCL) for the process. If all measurements lie within these limits it indicates the process is in control. The Central control limit (CCL) is calculated as the mean of the values, and is invaluable for further statistical analysis.

The "Rule of 7" states that seven consecutive results on one side of the mean could indicate a problem. Thus even when a control chart appears to be in control it could indicate a skew problem. 

The ASQ, highlights additional out-of-control indicators:

1) A single point outside the control limits

2) Two out of three successive points on same side of centerline and more than 2σ from it

3) Four out of five successive points on same side of centerline and more than 1σ from it

4) Eight points in a row on the same side of the centerline (Similar to the Rule of 7)

5) Also, obvious consistent or persistent patterns that could suggest something unusual about the data, process or the data gathering technique

As the name suggests, the process-behaviour chart is most efficient when results need to be predicted for an ongoing process. If the process is determined to be in control the control chart can be used to find and correct problems as they occur.

For process improvement the Shewhart chart can provide inputs to decide which specific problem points need to be addressed, or also if the complete process needs to be overhauled.

NB: A run chart is a "naked" form of control chart without the concept of control limits. The run chart simply displays process measurements over time.

Tool 2 - Check sheet

The Check Sheet is primarily a data collection tool that allows data being collected to be organized while it is being collected. This effectively means the data undergoes a preliminary analysis in realtime. The check sheet is best suited when several data points can be observed to collect data manually.

A clear understanding is required of what data points are to be observed and collected. These criteria are then specified in an organized table, creating the Check Sheet. Occurrence of each desired data point is then marked on the check sheet. The quickest analysis result possible is the count (frequency) of occurrence for each data point.

Consider an example where in an office survey, participants are asked to choose between 4 predefined options for which brand of coffee to use. The 4 predefined brands form the "predetermined data points" that need to be observed. As each participants answer is manually recorded on the check cheet, a realtime analysis of their answers starts to appear.

It is important to understand that a check sheet, is not the same as a check list, which is an inspection tool. The check sheet is most efficient when data being collected needs to be analysed based on frequency of occurrence. This of course indcicates that data collected in a check sheet can very effectively be represented using histograms.

For obvious reasons, a Check Sheet is also referred to as a "Data Collection Sheet" or a "Tally Chart".

Tool 1- Ishikawa Diagram

Named after  Kaoru Ishikawa, and also called the Cause-and-Effect diagram, the Ishikawa diagram is a brainstorming tool that engages in identifying causes for a particular problem. As the name suggests, it is a diagrammatic tool, where the process of braingstorming and thinking results in a diagram representation of the causes of a known effect.

There are two sections of the diagram; the right side has a box specifying the problem under analysis. A line is then drawn from the box extending towards the left. All possible causes suggested in the course of the brainstorming are then drawn as arms extending out from this main line. Causes can also be differentiated under different categories, with each category having it's own sub-branches. This gives the diagram a fishbone appearance, hence the diagram is also called a Fish-Bone diagram.

As a guideline, the ASQ recommends using the 6 Ms, Machine, Method, Materials, Maintenance, Man, Mother Nature (Environment), as basic categories for brainstorming. This of course does not prevent from identifying further categories.

Another technique to trigger brainstorming is "5 Whys" which involves repeatedly asking the question "Why?!" until the result is drilled down to specific, measurable, controlable causes.

The Fishbone diagram can be used when possible causes of a problem need to be identified, using peoples' thinking. Advantages of using Fishbone diagram are that the problem is clearly identified, multiple causes are identified and inherently classified, people are invovled in the generation of ideas and the output is a visual representation of the ideas.

The best situations to use the Ishikawa diagram are when root causes to known problem need to be identified by stimulating thinking and brainsotrming among a group of participants.

7 Quality tools - quick preview

Quality Control is defined as the "process of monitoring and recording the results of executing quality activities to assess performance and recomment necessary changes".

Quality Assurance, on the other hand, involves auditing results  obtained from the quality control measurements to ensure that the approriate quality standards and being used (PMBOK 4th ed.).

Achieving high quality, thus invovles a close interworking between Quality Control (gathering results from what we're doing) and Quality Assurance (studying the data gathered to make sure we're doing everything as decided).

The ASQ defines 7 tools that allow proper representation of measurements to facilitate further analysis and perform quality assurance and quality control.

1) Ishikawa diagram: used to identify as many possible causes for a problem

2) Check sheet: used to gather data, by human observation, usually in realtime

3) Control chart: this graph represents process behaviour over time

4) Histogram: this depicts the frerquency of occurance of each measurement value

5) Pareto chart: a form of the histogram where data is sorted in descending order

6) Scatter diagram: used to check for correlations between two sets of data

7) Stratification: a technique to seperate data into related groups

Several sources often replace Stratification with Flow Chart/ Run chart as one of the tools.

7+) Flow chart / Run chart: pictorial representation of methods used to achieve a result.

Although all the above 7 tools are used in varying degrees for representation and analysis of measurements, it is important to understand their unique capabilities and advanatages. Hence, selecting the right tool(s) depending on the situation is critical to derive highest efficiency from Quality Control and Quality Assurance.