Statistical process control, or SPC, is used to determine the conformance of a manufacturing process to product or service specifications. SPC relies on control charts to detect products or services that are defective. In manufacturing, the process is statistically monitored and controlled for deviations from the norm via random sampling to indicate whether production has changed. SPC seeks to measure the quality of work in process, and its implementation is typically the first step toward total quality systems management. However, this system has its disadvantages.
SPC emphasizes early detection and prevention of problems, which can be a benefit to the production of quality products. However, it takes time to apply rigorously in a manufacturing setting, as it requires more observations. Operators have the added responsibility of monitoring the process line and filling out charts, which is time consuming. Because it has to be integrated within your production system, you have to devote time to training operators and creating awareness in other personnel about the program. This may lead to downtimes as crucial staff have to attend these training sessions.
Implementation of SPC is a costly endeavor. In most cases you will need to contract a professional from a consultancy firm to conduct the training for your personnel, and these do not come cheap. Even if you opt for in-house training, you will need to spend money acquiring the necessary resources and materials and recruiting employees who are skilled in the quality function. Once implemented, you will start to experience a higher number of rejects on the production line, and these translate to losses.
SPC is typically understood to operate in a manufacturing context where production line machinery can be evaluated for conformance to process norms. Although SPC swiftly reveals when quality has changed, it does not show by how much the rejected products are defective. For example, it does not provide an integer number for the out-of-tolerance dimensions on product pieces, which would require precise measurements. Nor does it indicate the direction and magnitude of corrections.
SPC relies on operators to provide data about the process using the charts they fill while monitoring the production line. Its implementation can be taken as an additional task by the production personnel. In addition, although a high number of rejects on the production line is an indication of an out-of-control process, this may be perceived as implying criticism of an employee's capacity to handle the job. Unless management has explained the role of SPC fully, some workers may be intimidated and insecure about their jobs and be unwilling to report problems.