Project Quality Management: Control Limit vs Specification Limit for PMP Exam
Control charts are one of the Seven Basic Quality Tools used in Control Quality as described in PMBOK® Guide to give a visual depiction of the changes in the outcome of a process. Though in the ideal situation, a process will give the same results for each and every time it runs. However, in reality, the results will be different from each other owing to a number of factors (common causes and special causes). The Control Limits and Specification Limits are a threshold for evaluating when the process is under control or not.
This post will expound on the similarities and differences of Control Limit vs Specification Limit and what Aspirants would need to know for the exam.
Control Limit vs Specification Limit
- Control Limit: the limit established for the control chart based on statistical analysis or from historical records
- There are 2 Control Limits: Upper Control Limit (ucl) and Lower Control Limit (lcl) indicating the maximum and mininium allowable values respectively
- By convention, the Control Limits would usually be±2 or ±3 standard deviations (σ) from the target value, though this will vary from process to process
- If there are results falling outside the Control Limits, the process is said to be unstable (i.e. out of control) ; root analysis is needed to investigate the special cause and rework/scrap would be needed
- In addition, there are still some other pattern that will indicate the process is out of control, e.g. 7 consecutive data points on either side of mean (Rule of Seven)
- Specification Limit: the allowable deviations requested based on customer expectations
- There are 2 Specification Limits: Upper Specification Limit (usl) and Lower Specification Limit (lsl) indicating the maximum and mininium allowable values respectively
- If there are results falling outside the Specification Limits (but within Control Limits), improvement works would be needed to adjust the process to give greater precision; nevertheless the process is still considered stable
- The Specification Limits would be documented in agreements and penalties would be imposed if the results fall outside these Specification Limits
- Specification Limits may be within or beyond the Control Limits — that would depend on the precision that is required for the process
Control Limit vs Specification Limit Illustrated
Let’s take the project of PMP® study and preparation as an example to illustrate the concept of Control Limit and Specification Limit.
When working on some mock exam papers, your score for 5 different mock question sets were:
The mean for the above 5 mock exam paper results is 70.4% which is above the minimum recommended score (70%) for mock exam results and all these scores were within the control limits of 2σ (74.5% and 66.3%). So the the Aspirant is said to have a stable performance and is expected to be able to pass the exam in first try.
Though all the scores are within the control limits (64.5% and 55.3%), the mean of 60.4% is below the recommended minimum score. Though the PMP® Aspirant has a stable performance, he/she will likely to fall the real exam.
The mean here is 70.3%, which is above the minimum recommended score (70%) for mock exam results, however, the exam results with 100% is well beyond the control limits (96.4% and 44%). Action would be needed to find out the root cause for getting the 100% results (e.g. is it just too easy?).
The PMP® Aspirant would like to place a Lower Specification Limit of 70% for all the mock exam results as a threshold for writing the real exam. From the results of the above 3 scenarios, there are quite a number of results that are lower than this Specification Limit and the Aspirant will need to study harder and bridge his/her knowledge gap before attempting the real exam. If he/she has booked the exam, he/she will need to postpone the exam date then.
Summary: Control Limit vs Specification Limit
Aspirants would need to remember Control Limits are there to indicate whether the process/system is under control or not. Results falling outside the Control Limits would mean the process is unstable and root cause analysis is needed. Specification Limits are imposed by agreement with customers on strictly quality requirements, i.e. Specification Limits must be within the Control Limits.
Hope this article can illustrate the differences between Control Limit vs Specification Limit well.