What is the difference between Control Limits and Specification Limits?

tarting with the basics,

Control limits are the process's voice (what the process does) and

Specification limits represent the customer's voice (what we want the process to do).

Lower and upper control limits are LCL and UCL, respectively.

Lower and upper specification limits are LSL and USL.

In general, these limits represent our process variation and help highlight when our process is out of control.

x̄ - center line for the data is the sum of all the input data divided by the total number of data points

LCL – is 3 process standard deviation below the average and

UCL – is typically 3 process standard deviation above the average

If the data point falls within ±3SD of its average, it is considered as “EXPECTED” behavior for the process and thus is a common cause variation. Something special happened to those data pointers outside these control limits and can be special cause variation.

Let’s consider real-time data from the 2022 Japanese Grand Prix. The qualification session usually determines the starting order and the pole position of the race. To occupy pole position, the fastest driver must ensure that their performance is the quickest, that is, with the lowest lap timing.

Below are the lap time/s (actual performance) for the qualifying session.

LCL 01:29.304

UCL 01:31.511

Five of the slowest cars are eliminated.

Now comes the specification limit, which is set based on the 107% rule.

For instance, if the fastest lap time was 100 seconds, each driver who is eliminated in the session must complete at least one lap within 107 seconds to guarantee a race start, which is the USL. Often, only one specification limit is used as in this example.

Control limits are applied to summary statistics, whereas specification limits are applied to individual measurements. Control and specification limits are extensively used in control charts and can give us an early warning if a process is showing irregularities, giving us the opportunity to take remedial steps before the situation becomes a problem.

Let’s consider another example from the customer contact center: if the average handling time is 4 minutes with a standard deviation of 1 minute, then the control limits are UCL = 7 minutes and LCL = 1 minute, respectively. Specification limits are the targets for the process and defined by the customer or based on the performance of the market. It is desired that those control limits be within the specification limits so that, in case of special causes, the customer will not be impacted.

Application of control limits in control chart:

The position and scattering of data points plotted on the control chart assist us in identifying process behavior. Process behavior includes that of identifying the stability and understanding the pattern of process variation from a special and common cause viewpoint.

Let’s consider one more example based on the below referred Shewhart chart of a manufacturing unit of Prod X.

If we could notice that none of the data points are outside the specification limits, and usually the production management team will be OK and glad about this type of run, it is worth noting that if the process were managed statistically, these patterns would assist the line engineer in adjusting and retargeting the process. However, the effectiveness of re-targeting will be determined by the usefulness of the process gain factors.

Consider the following scenarios to better understand process performance:

Scenario 1: The specification limit is within the control limits.

Scenario 2: The specification limit is the same as the control limit.

Scenario 3: The specification limits exceed the control limits.

In scenario 1, part of the process (natural variation) may function outside of customer-defined targets, leading to rejections and defects.

In scenario 2, the process may meet customer specifications, but it can produce defects when there is an uncommon source of variation.

In scenario 3, which I would call an optimal scenario, the production is stable, within its capability, produces no defects or rejections, and of course meets the customer specification.

To summarize,

Control limits are calculated from the process data, so they are the voice of the process, and specification limits are defined by the customer, so they are the voice of the customer.

Control limits emphasize location, spread, and width, whereas specification limits focus on meeting the requirements.

Control limits are applied to subgroups, and specification limits are applied to items.

Control limits help reduce internal rejections in the process, and specification limits help reduce customer rejections.

Control limits are displayed in the control chart, and specification limits are displayed in the histogram and probability plots.