SPC introduction

SPC brief introduction

Statistical process control （Statistical Process Control） It is a process control tool with the help of mathematical statistics . It analyzes and evaluates the production process , According to the feedback information, timely find the signs of systemic factors , And take measures to eliminate its impact , Maintain the process in a controlled state only affected by random factors , To achieve the purpose of quality control .

Technical principle

Use statistical methods to monitor the status of the process , Make sure the production process is under control , To reduce the variation of product quality . It thinks , When the process is only affected by random factors , The process is under statistical control （ Controlled state for short ）; When there are systematic factors in the process , The process is statistically out of control （ Out of control ）. Because the process fluctuation has statistical regularity , When the process is under control , The process characteristics generally obey the stable random distribution ; And out of control , The process distribution will change .SPC It is the statistical regularity of process fluctuation that is used to analyze and control the process . thus , It emphasizes that the process runs in a controlled and capable state , So that the products and services can stably meet the requirements of customers .

The implementation of SPC The process of is generally divided into two steps ：

1、 use SPC Analyze the process , Such as drawing control chart for analysis, etc ; Take necessary measures according to the analysis results ： It may be necessary to eliminate systemic factors in the process , It may also require management intervention to reduce the random fluctuation of the process to meet the needs of process capability .

2、 Monitor the process with control chart .

Control chart is a graphical method , It gives the information of the sample sequence representing the current state of the process , This information is compared with the control limit established after considering the inherent variation of the process . The control chart method is first used to help evaluate whether a process has reached 、 Or continue to maintain a state of statistical control with an appropriate specified level , Then used to help in the production process , By maintaining continuous product quality records , To obtain and maintain the control and high consistency of the characteristics of important products or services . Apply the control chart and carefully analyze the control chart . Can better understand and improve the process , That is to realize the SPC Process control .

According to the purpose of the control chart , Control charts can be divided into ： Control chart for analysis and control chart for control . Depending on the type of Statistics , Control charts can be divided into ： Metering control chart and counting control chart （ Including piece control chart and point control chart ）. They are applicable to different production processes . Each class can be subdivided into specific control charts , Initially, there were mainly seven basic charts .

Metering control charts include ：

* IX-MR（ Single value moving range diagram ）

* Xbar-R（ Xbar-R Chart ）

* ME-R（ Median range chart ）

* Xbar-s（ Mean standard deviation plot ）

The counting control chart includes ：

* P（ It is used for the rate of nonconforming products with variable sample cost ）

* Np（ Number of nonconforming products used for fixed sample size ）

* U（ Number of defects per unit used for variable sample cost ）

* C（ Number of defects for fixed sample size