Specification limits are the targets set for a product or the process by customer or market performance; often, Voice of the Customer is the input for customer specification limits. 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 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.
Two types of Specification limits
Upper Specification Limit: The highest limit a customer would accept.
Example: A customer would wait in line at a drive through 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?
Customer defines the specification limits where the losses due to variation is equals the benefit of the product. Generally, these values drawn on the histogram. If the product falls between the USL and LSL, then the product is meeting the customer requirement. Specification limits determines the process capability and the sigma value.
Process capability indices
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 meet Customer CTQs (requirements).
Cp, and Cpk values helps to determine the Process Capability. Generally you use this when a process is under statistical control. Whereas Pp and PPk are for Process Performance. Generally you use this when 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 the standard 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 & 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 “Voice of process”.
Specification limit is different than control limit. Customer defines these values, hence it is called ” Voice of Customer”.
Usage of Specification limits and Control limits
- Specification limits and control limits cannot draw on the same graph. However, it helps to determine the stability and capability of the process.
- There is no direct relationship between specification limits and control limits. However, they can be compared.
Case1: Specification Limit falls within Control Limit.
If specification limits fall within the control limits, part of the process may function outside the customer set range. So, the process is producing the defects.
Case 2: Specification Limit Coincides with Control Limit
If specification limits fall on control limits, the process variation is common and process may meet customer specification. However, it may produce a defect because possibility of influence with uncommon source of variation.
Case3: Control Limit falls within 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 the production. Also process meets the customer specification.
Can we use specification limit and control limit together?
No, Specification limit and control limit cannot used together because both are completely different to each other. Control limit helps to assess whether the process is stable. While, customer defines the specification limit.