Specification limits are the targets set for a product or the process by customer or market performance; often, the Voice of the Customer is the input for these criteria. In other words, it is the intended result of the metric that we measure.
Customers set the limit (upper and lower) on the product characteristics that define where the product works and where it does not. If the product falls outside of these limits, assume that the customer will reject the product. Specification limits are related to product design. Hence, set these limits in the design phase of the product life cycle.
The Two Types
Upper Specification Limit: The highest limit a customer would accept.
Example: A customer would wait in line at a drive-through for 7 minutes before being dissatisfied.
Lower Specification Limit: The lowest limit a customer would accept.
Example: A customer would expect no less than 100 M&Ms to be in a packet before being dissatisfied.
Why Specification Limits?
The customer defines limits where the losses due to variation are equal to the product’s benefit. Generally, these values are drawn on the histogram. If the product falls between the USL and LSL, then the product meets the customer’s requirement. In other words, they determine the process capability and the sigma value.
What are Natural Process Limits
Process control limits, also known as Natural process limits help to monitor and manage the inherent variability of a process. The process control limits are typically derived from statistical analysis of historical process data and are used to identify whether a process is in a state of statistical control.
Process control limits are derived from statistical analysis of historical process data. Typically defined as the mean (average) ± a certain number of standard deviations. Process variations within these limits are considered normal and expected.
Cp and Cpk are called Process Capability. Pp and Ppk are called Process Performance. In both cases, we want to try to verify if the process can meet to meetCustomer CTQs (requirements).
Cp and Cpk values help to determine the Process Capability. Generally, you use this when a processis under statistical control.Whereas Pp and PPk are for Process Performance. Generally, you use thiswhen a process is too new to determine if it is under statistical control.
Cp (Process Potential)- Tolerance width divided by process capability = (USL – LSL) / (6*σ)
Cpl and Cpu are the one-sided capability ratios Cpl = (µ-LSL) / (3*σ); Cpu = (USL-µ)/ (3*σ), µwhere is the target for the process
Cpk is the process capability index Cpk = min (Cpl, Cpu)
Pp (Process performance) = (USL – LSL) / 6* s, In Pp, s is thestandard deviation
Ppl=(xbar- LSL)/ 3s ; Ppu= (USL – xbar)/ 3s, where xbar is the Process Mean
Ppk is the process performance index Ppk=min(Ppl, Ppu)
Difference between Specification limit and Control limit
Control limits are the limits of variation that are expected from a process when the process is in statistical control. Furthermore, control limits are calculated from the process data, so it is called the “Voice of process.”
The specification limit is different than the control limit. The customer defines these values; hence it is called the “Voice of Customer.”
Usage of Specification limits and Control limits
You cannot draw both on the same graph. However, it helps to determine the stability and capability of the process.
There is no direct relationship between them. However, they can be compared.
Case 1: Specification Limit falls within Control Limit.
If specification limits fall within the control limits, part of the process may function outside the customer’s set range. So, the process is producing defects.
Case 2: Specification Limit Coincides with Control Limit
If specification limits fall on control limits, the process variation is common, and the process may meet the customer’s requirements. However, it may produce a defect because of the possibility of influence with an uncommon source of variation.
Case 3: The Control Limit falls within the Specification limit
If control limits lie within the specification limits, production is stable within our capability. It produces the best design and no major issues in production. Also, the process meets the customer’s specifications.
Can we use the specification limit and control limit together?
No, they cannot be used together because they are completely different from each other. Control limit helps to assess whether the process is stable, while the customer defines the specification limit.
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Ted Hessing
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Specification limits are boundaries set by a customer, engineering, or management to designate where the product must perform. Specification limits are also referred to as the “voice of the customer” because they represent the results that the customer requires.
Specification limits are the values between which products or services should operate. These limits are usually set by customer requirements. For example, you print labels for a shipping process. If the labels are too big or too small, they will not feed into printers properly.
Specification limits are used to determine process capability and sigma value. You can define Upper Specification Limits (USL) and Lower Specification Limits (LSL) for each chart. Control limits are limits or averages that are calculated based on previous history.
The area, on either side of the centerline, or mean, of data plotted on a control chart that meets the customer's requirements for a product or service. This area may be greater than or less than the area defined by the control limits.
Specification limits are the targets set for a product or the process by customer or market performance; often, the Voice of the Customer is the input for these criteria. In other words, it is the intended result of the metric that we measure.
LSL (Lower Specification Limit) - The value three standard deviations below the Mean, or Target, of the Output. USL (Upper Specification Limit) - The value three standard deviations above the Mean, or Target, of the Output. Target - The Target, or Mean, outcome of the distribution.
Specification limits are boundaries set by a customer, engineering, or management to designate where the product must perform. Specification limits are also referred to as the “voice of the customer” because they represent the results that the customer requires.
A tolerance is the accepted parameters for a specification. For example, the specification calls for a 2cm wide laser cut hole through a piece of aliminum, but you can accept 1.95cm to 2.05cm width . . . tolerances indicate an acceptable range, it's really determined by criticality.
Definitions: Specification Limits are the targets set for the process/product by customer or market performance or internal target. In short, it is the intended result on the metric that is measured.
Any value beyond the specification limit indicates a defect or unacceptable result. The farther the specification limits are from the mean, the lower the chance of defects. A Six Sigma process has a specification limit which is 6 times its sigma (standard deviation) away from its mean.
UCL, LCL. The Upper Control Limit (UCL) and the Lower Control Limit (LCL) form a corridor within which the quality characteristic meets the desired value or a common cause of variation (Figure 7.7).
The setting of specifications for drug substance and drug product is part of an overall control strategy which includes control of raw materials and excipients, in-process testing, process evaluation or validation, adherence to Good Manufacturing Practices, stability testing, and testing for consistency of lots.
Specification limits come from your customer. They are limits on product characteristics that define where the product works and where it does not work. Spec (Specification) limits define the allowable deviation from a target or ideal value.
Control limits are based on the natural variation of your process, while specification limits are based on the expectations and requirements of your customers. Aligning these two types of limits can help you reduce defects, optimize performance, and increase customer satisfaction.
Generally there are two types of specification limit i.e. LSL (Lower Specification Limit) and USL (Upper Specification Limit) These limits quantify the requirement of customer (CTQ).
Upper Specification Limit: The highest limit a customer would accept. Example: A customer would wait in line at a drive-through for 7 minutes before being dissatisfied. Lower Specification Limit: The lowest limit a customer would accept.
Include specs such as dimensions, safety standards, expiry details, and an overall product design specification. These specs get product creation underway more easily. You might leave other specifications, such as specific UI/UX considerations or color schemes, to be defined later in the product development backlog.
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