First, let’s start with some simple definitions:
Statistics- is the study of the collection, analysis, interpretation, presentation, and organization of data.
Statistical tools – is the application of statistics for the purpose of visualizing, identifying and predicting results based on the data collected.
Statistical Process Control (SPC) – is the recording and measuring of the parameters of a process such as speed, pressure, caliper etc against a set of standards using statistical methods to verify they are within required limits. Objective: Minimize variation and run to optimum target
Statistical Quality Control (SQC) – is typically the measuring and recording data against specific requirements for a product ensuring they meet the necessary requirements – size, weight, colour etc. Objective: Auditing process validating outputs from a process meet the requirements of the ultimate customer or next stage of the manufacturing process.
Some believe that both items fit under one title Statistical Process Control (SPC). That very well may be true but in a typical manufacturing facility there are two distinct rolls. Production makes the products and Quality Control/Assurance verifies the products manufactured meets the requirements – an auditing role.
While the two roles exist in a facility there is a move to do more of the SQC validation directly to the manufacturing floor decreasing the time lag between finding a problem and fixing it.
The important part of both of these responsibilities is to ensure that both roles are being performed and more importantly checking the right parameters and measuring on the right frequency. This is where Statistical tools come into play and are very important. Applying statistical tools to the data collected allows for the detection of immediate issues like being outside specification or control limits. These would be detected based on the setting of these limits and measuring against them. This is done through the use of Alarm type screens like the Summary Screen In QW 6. It can also be observed using visual tools like the Control Chart screen in QW 6.
The next set of statistical tools involve what is termed descriptive statistics. Descriptive statistics are applied to a population of data and are used to describe the data in that population. – statistics like Average, Mean, Standard Deviation. QW 6 provides an extensive list of these Statistics to select from to best suit any population of data collected in a QW 6 application. A key understanding of statistics is that they act indicators, like blood pressure and heart rate, to help diagnose or better understand the data collected.
“Statistics are valuable representations of data that assist in the analysis and decision making process”
Another valuable tool is Inferential Statistics which are still based on a population of data like Descriptive statistics but “speculate” based on that population. An example would be the Descriptive statistic Observed Out of Specification where each data point in the population is measured against fixed specification limits to determine the number that exceed the specification limits. The inferential example would be the Calculated out of specification which based on the volatility of the data speculates if there would be more samples out of specification. In other words should there be more samples taken to ensure you are not making defects.
Blending the use of these two types of statistics along with intuitive visual presentations of data combine to provide a very powerful continuous improvement tool for the shop floor.
So in summary SPC is focused on minimizing variation in a process and running at target while SQC, while using similar tools, is the auditing method of insuring outputs meet exact requirements. QW6 provides the tools to meet both these objectives and promotes the sharing of data to minimize the duplication of time for the collection of the data.