Quality control charts also called the Shewhart chart are used as an important part of process improvement techniques in numerous industries. Variations within any process are an unavoidable aspect. But when timely corrective action is not taken then these variations can affect the business negatively. The importance of quality control charts is, in such scenarios, to study the changes happening over time in the process.
Control charts are graphs with a central line for the average, an upper line for the upper control limit, and a lower line for the lower control limit. Control charts in quality control are used as a tool for data collection, data comparison and analysis. For variable data, these charts are used in pairs whereas for attribute data they are used singly.
What is a control chart in quality control?
In quality control, control charts are always used in combination with other process improvement tools. Across industries such as healthcare, service, manufacturing or others, control charts can be used to monitor how variables change over time. Using quality control charts effectively is all about having a good understanding of the variations and enabling continuous improvement.
A control chart in quality control is especially useful to monitor the process that is under statistical control. Quality control charts are considered one of the seven basic quality tools for process improvement and are used in Lean Six Sigma projects and DMAIC (an acronym for Define, Measure, Analyze, Improve and Control) projects that are under the control phase.
The importance of quality control charts is to enable the visualization of variation, monitor for problems and take actions to improve processes, predict expected outcomes and also to analyze the variation patterns. For optimum quality control, it is crucial to identify the right control charts on the basis of the available data.
Importance of quality control charts
Quality control charts are one of many graphical tools used in quality control analysis to understand the process changes that occur over time. The importance of quality control charts is evident in their use as statistical quality control tools. Statistical control charts are used to determine variables, ascertain unit defect fractions, find out the faults per unit and also as a procedure for acceptance sampling. It is not feasible for the manufacturer to inspect the entire batch of products for quality control, and this makes sampling necessary.
Here are some benefits of control charts that aid in quality control:
- Determine process variations that occur within and outside of your control limits.
- Find problem indicators, and know beforehand if something may or may not go wrong in the process.
- Determine patterns within plotted points that could be the possible causes of variations.
- Guide in deciding the right solutions or corrective actions to be taken.
- Forecasting future performance.
- Find new ideas to improve quality on the basis of analysis from quality control charts.
Types of quality control charts
There are different types of quality control charts and each of these was developed to suit particular characteristics of the quality attribute being analyzed. These differences among the charts are depending on whether the nature of the monitored data is variable or attribute.
P chart in quality control
P charts in quality control are used when the data to be monitored is counted and when the sample size can change over time. Each of these data points is a ratio of its own discrete sample set. P charts can show the fraction or percentage of nonconforming data values and if this proportion changes during the sampling period.
NP chart in quality control
NP chart in quality control is similar to the P chart. The difference is that the sample size will need to stay constant during the sampling period. These charts are the best when it comes to presenting the number of non-conforming or conforming items. In this chart, the number rather than the fraction of non-conformances is recorded.
C chart in quality control
C chart in quality control is used to monitor and control data that is in the form of specific numbers.This can include data such as the number of defects in a batch of raw material inspection or the number of defects in finished products. C charts are best suited when the number of defects per sample unit and the number of samples per sampling period remains constant. C charts show non-conforming or conforming items within a consistent unit size and in a specific period of time.
S chart in quality control
The S chart in quality control is also called the Standard Deviation chart. This control chart is used for monitoring data that is variable and when large numbers of samples need to be recorded. By using the standard deviation all data within a sample set are utilized to determine the variation and not just the minimum and maximum values.
X chart in quality control
The X chart in quality control is used to monitor the statistical mean or average of a variable in a set of samples. These charts can then be utilized to find the actual process mean, versus a nominal process mean. The X chart shows variable data and also if the mean output of the process undergoes any change over time.
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