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Elements of measurement

The Quality Toolbook > Measurement > Elements of measurement

Two types of measurement | Types of numeric data | Components of measurement


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Measurements may be more successfully selected and applied if their different parts and classifications are first understood. This section looks at aspects of measurements that may be taken into account when using them.

Two types of measurement

Information gathered may be of two broad types: quantitative or qualitative. Each has its value, but they may be gathered and treated quite separately.

Quantitative information

The easiest type of information to measure and use is a numeric quantity. Numbers are precise and help clear decisions to be made. Tools which use quantitative data often work by using a combination of mathematical calculation and comparison of numbers against one another or against a fixed and critical value.

Most of the discussion in this chapter is about quantitative information.

Qualitative information

Quantitative data is not always available and not always enough. Qualitative information is non-numeric, typically appearing as written text. This often comes in 'chunks', where a phrase or sentence describes a single, independent piece of information. Tools which use qualitative data typically organize and structure these chunks relative to one another, thus revealing further information.

Often, a situation is best described by a combination of numeric and non-numeric information, where the qualitative text helps to put the quantitative numbers into context, for example describing who was using a machine, where, under what conditions, etc.

Types of numeric data

When identifying what to measure, two main types of data should be taken into consideration, as each has applications where it is more useful.


One of the simplest measurements that can be made in many situations is to count the number of items in a particular classification, such as the number of customers purchasing full insurance cover or the number of defects in each sheet of glass. This attribute measurement answers the question, 'How many?' and its simplicity often makes it a good starting point, with variable measurement being used when the problem area has been more narrowly identified.

Attributes are a good way of turning qualitative data into quantitative data, for example by counting employees who think they are significantly underpaid.


Beyond attribute measurement is variable measurement, where the question, 'How much?' is asked. Variable measurements have units, such as centimeters and kilograms. They also usually require more effort to collect than attributes, and the actual measurement usually requires the use of some form of measuring instrument.

Components of measurement

In any measurement, there are several components that must be taken into account when deciding what to measure.


The measurement will be made in some kind of units. These should reflect the range of possible values, for example it is probably better to measure the length a piece of wood in millimeters rather than centimeters. Clearly stating the units to be used prevents situations where different people use different units and cause confusion in calculations and displays.


Many measurements are made in the form of numbers, as this is an absolute and flexible format, and the scale is simply the possible range of measured values. In some situations, however, numbers are not so useful, for example when identifying the possible actions of a customer upon finding a defective product.

In this case, the measurement scale is typically made up of a defined and discrete set of values. This can be useful when numbers are unclear, and it is easier to describe your satisfaction as 'high' or 'low' rather than '1' or '5'.


There is often a target or goal value for the measurement. This may be a center value about which the measurement varies, or a distant target that is to be achieved, as in the figure below. The measurement can thus be usefully expressed as a difference from this ideal, rather than as an absolute value.


As well as a target value, there are often some kind of action limits, beyond which the measurement should not go. If the measured value falls outside such specification limits, then some kind of action may be defined, such as rejection of the measured item or an investigation into the cause of failure. On the other hand, it is desirable to beat target values.


The measurement may be made using some kind of measuring device. It is essential that this tool is accurate and reliable, as an uncertain measuring tool will result in worthless measurement values. Measurement tools include all methods of gathering data, from voltmeters to surveys. Each has constraints in use and the data given must be of a known accuracy to enable confident decisions to be made.


If the measurement process is clear and well defined, then each measurement can be made in a consistent way, enabling successive values to be compared. Detailing the process also puts into perspective the actual work that has to be done to collect the data, and enables the requisite time and resource to be scheduled.

Details of the measurement process may include:

  • Who is doing the measuring and how you will be sure they know what to do.
  • When the measurement is done, including times, events and frequencies.
  • What is to be measured, including items and sampling rules.
  • The tools to be used, including calibration details.
  • How the measurements are recorded, including design of Check Sheets.
  • What is to be done with the completed data, including storage and actions.


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