What is Process Capability? Objectives, Measurement, Process

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What is Process Capability?

Process Capability refers to the ability of a process to consistently produce output that meets the customer’s specifications or requirements. In other words, it is a measure of how well a process is performing in terms of its ability to produce products or services that meet or exceed the customer’s expectations.

Process capability is usually measured using statistical methods such as process capability analysis, which calculates various parameters such as Cp, Cpk, Pp, and Ppk. These parameters indicate how well the process is centered on the target value, how much variability there is in the process output, and whether the process output is within the specification limits.

Concept of Process Capability

The term ‘process’ defines a logical method of converting a set of inputs into a desired output. It includes the tools, techniques, and methods that convert an input into an output.

The term ‘process’ defines a logical method of converting a set of inputs into a desired output. It includes the tools, techniques, and methods that convert an input into an output.

It should be noted that every process shows a natural variation in the process execution and sometimes abnormal variation due to some special causes.

Consider an example of a pizza chain that promises to deliver a pizza to its customers in 30 minutes; else the pizza will be free of cost. This objective can be met if the process capability of the pizza chain is excellent. Here, 30 minutes is the constraint that the pizza chain has to follow. There may be instances when the pizza chain fails to meet this commitment due to random or abnormal variations, which reduce or damage the process capability. Let us consider another example. You regularly visit a restaurant for a lunch.

You give your order to a waiter, who passes it to the kitchen. Your lunch is usually served within 15 to 20 minutes. However, lately, you are not happy with the service of your restaurant. Waiters take their time in getting your order, and the kitchen staff is also late in serving you the lunch. Despite your complaints to the manager, there has not been any action to improve the service. Finally, fed up with the poor service and incapable process of the restaurant, you stop visiting it. What is the outcome? The restaurant loses a loyal customer, and within a couple of months closes down due to declining business.

This example shows the capability of a process has widespread impacts on the business. If the management had taken some corrective actions based on the customers’ complaints, then the service of the restaurant would have been improved, thereby improving the process capability and saving the restaurant.

Objectives of Process Capability

The main objectives of process capability are as follows:

To Make Operations System Dependent Instead of Person Dependent

Process capability streamlines day-to-day operations and moves their dependency to a system, instead of a person. Therefore, when an organization follows a process-based culture, it does not suffer due to the whims and fancies of a person.

For example, an experienced operator in a factory goes on leave. However, there are adequate Standard Operational Procedures (SOPs) in the factory so that another worker can easily replace his place in the assembly line. This ensures the continuity of business operations.

To Increase Productivity

Process capability increases the productivity of an organization. Consequently, the organization can finish more work within a specified amount of time, as the processes move in a more systematic manner. The result is large outputs in less time.

To Improve the Quality of Products

Process capability delivers high-quality products or services, which satisfy all requirements specified by customers. This is because process capability ensures that an organization correctly captures the requirements of the customers are delivers the products according to the specifications.

To Improve Communication

Process capability enhances the communication mechanism within and across an organization. It helps the organization to implement effective and efficient modes of communication channels.

Measurement of Process Capability

Process capability measurement aims to find the standard deviation of the individual measurements of products when a process is in control.

  • Its specification tolerance
  • Whether the process average is centered mid-way between the tolerance limits
  • Measurement of inherent (piece-to-piece) variability of the process
  • Measurement of actual variability over a period
  • Causes of differences between inherent and actual variability

To analyze the process capability, you can use a control chart or a frequency distribution method. During this method, make sure of the following conditions:

  • Use homogeneous material to minimize abnormal material variation
  • Do not make any process adjustments during the study
  • Allow only trained operators to work

The steps involved in the control chart or frequency distribution method are as follows:

  • Take a number of samples over a period, where each sample consists of consecutively made pieces.

  • Calculate the average mean ) and range (R) of each sample.

  • Calculate the total average of sample means. This measures the center-line of the process.

  • Calculate the Upper Control Limit (UCL) and the Lower Control Limit (LCL) and plot X and R charts.
    This measures the stability of the process, that is, the extent to which it changes with time.

  • Calculate the process capability 6σ = 6R/d2. This measures the inherent variability of the process.

Process Capability Index

A process capability index is an analytical measure that compares the performance of a product or a process with its predefined specification. This index reduces the complex information about a process into a single value, which helps an organization to compare how the process fares in the eyes of the customer.

The process capability index helps you to easily evaluate the effect of a process before and after a change. This index also indicates the expected future performance of a stable process. The capability of a process is defined as the ratio of the distance from the center of the process to the nearest specification limit divided by a measure of process variability.

There are two process capability indicators, which are commonly used in the optimization process: CP and CPK.


It is defined as the relation between the specification range and the process width i.e. the spread of the distribution. The formula for Cp is as follows:

Cp = S / P

Here S = specification range = U – L = difference between upper and lower tolerance

P = process width= 6σ

  • If C = 1, then process capability is fine
  • If CP > 1, then process capability is bad
  • If CP > 1, then process capability is good


This process capability indicator considers the inconsistencies that are observed in Cp

The formula for Cpk is as follows:

Process capability = min(USL-µ/3σ, µ-LSL/3σ) Here,

USL = Upper Specification Limit

LSL = Lower Specification Limit

µ = Process mean

σ = Standard deviation

To understand the difference between Cp and Cpk, consider the example of a bowler in a cricket match. If the bowler’s group of deliveries land on the same spot, for example, outside the off stump of a batsman consistently forming a good group, then this is high Cp. Now, the bowler lightly adjusts his length and line and bowls his next group of deliveries on the middle stump, and with each delivery, he uproots the stump and takes a wicket. So now the bowler has high Cpk.

Apart from this, there are certain other process capability indices, which are shown in Table 4.2:

CpProcess capability for two-sided specification limits, which does not consider where the process is centered (i.e., what is the process average (X).
Cp,nProcess capability based on the USL.
Cp,lProcess capability based on the LSL.
CpkMinimum of Cpu, CplProcess capability for two-sided specification limits, which considers where the process is centered

Among these indices, process capability indicator Cpk is an excellent measure of process capability and variability because it considers both the spread and the non-centering of the apparent shift in the process variation. In process control, centering a process is much easier than reducing the spread. Centering requires a relatively simple adjustment, whereas reducing the spread requires a great amount of planning and redesign.

Comparison of Process Capability With Design Specifications

Process capability is the ability of a process to meet design specifications for a product or service. Design specifications are usually in the form of a target and a tolerance. They are established according to customer requirements. They indicate a range of values in which individual units of output must fall in order to be acceptable.

In process capability analysis, you determine whether the inherent variability inherent in the output of a process that is in control falls within the acceptable range of variability allowed by the design specifications for the process output. If it is within the specifications, the process is said to be ‘capable’. If it is not, you must decide how to correct the situation.

To give a simple analogy, in process capability you determine what are the limits of a process that is in control and find if there are any outlines (voice of the process). Then, you define what the limits are and see how the data fits into them (voice of the customer). If it does not, then you change the process till the data fits into it.

The three cases are:

  • Case A: Process capability and output specifications are well matched. Therefore, almost all the process output can just match the specifications.

  • Case B: Process variability is well within specifications. Nearly 100% of the output must match the specifications.

  • Case C: The specifications are tighter than the process capability. Therefore, even if the process is functioning as expected, a considerable percentage of the output will not meet the specifications. In other words, the process could be in control and still generate unacceptable output.

Therefore, you cannot automatically assume that just because a process is in control, it will provide the desired output. Instead, you must specifically verify whether a process is capable of meeting specifications and not simply set up a control chart to monitor it. A process should be both in control and within specifications before production begins.

In Case C shown in Figure, you might consider the following possible solutions:

  • Redesign the process so that it can achieve the desired output.

  • Use an alternative process that can achieve the desired output.

  • Retain the current process but try to remove unacceptable output using 100% inspection.

  • Examine the specifications to see whether they are mandatory or could be relaxed without harming customer satisfaction

To determine whether a process is capable, you can compare ±3 standard deviations (i.e., 6 standard deviations) of the process to the specifications for the process.

For example, suppose the ideal length of time to perform a service is 10 minutes, and an acceptable range of variation around this time is ±1 minutes. If the process has a standard deviation of .5 minutes, it would not be capable because ±3 standard deviations would be ±1.5 minutes, exceeding the specification of ±1 minute.

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  • What is Process Capability? Retrieved from http://www.itl.nist.gov/div898/handbook/pmc/section1/pmc16.html

  • Process Capability Statistics: Cp and Cpk, Working Together. Retrieved from http://blog.minitab.com/blog/statistics-and-quality-improvement/process-capability-statistics-cp-and-cpk-working-together
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