Take samplesCheckoutCountermeasuresNormalprocessDraw Controlcause analysisChartAbnormalprocessWhetherNoYesthe processis abnormal2007.4The implementationcycle of control chart16
2007.4 The implementation cycle of control chart Take samples Checkout Draw Control Chart Whether the process is abnormal Normal process Abnormal process cause analysis Countermeasures No Yes 16
Normalityassumption3oruleSmallprobabilityprincipleThe thought of proof by contradiction2007.4The design principle of control chart17
The design principle of control chart 2007.4 3σrule Normality assumption Small probability principle The thought of proof by contradiction 17
Normality assumptionThere will always be a certain degree of volatility forthe quality characteristic value of the products whichare produced by any production process,when theprocess is stable or controlled, these fluctuations aremainly random errors that are caused by smallchanges in the 5 MiE. At this moment,the assumptionthat most of the quality characteristic value obey orapproximately obey the normal distribution is callednormality assumption.5MIE:Man,Machine,Materials,Technologicalmethod,Measurement and production environment2007.4Normality assumption18
Normality assumption 2007.4 Normality assumption There will always be a certain degree of volatility for the quality characteristic value of the products which are produced by any production process,when the process is stable or controlled, these fluctuations are mainly random errors that are caused by small changes in the 5 M1E. At this moment,the assumption that most of the quality characteristic value obey or approximately obey the normal distribution is called normality assumption. 5MIE: Man,Machine,Materials,Technological method, Measurement and production environment 18
3oruleWhen only accidental error exists in the productionprocess,the quality characteristic X obeys normaldistribution N( μ,o ),and according to theprobabilistic properties of the normal distribution,there isP(μ-3c< X<μ+3 c ) =99.73%Thatis, （ μ - 3 o,u + 3 ) is the practical valuerange of X.2007.43orule619
3σrule 2007.4 3σrule When only accidental error exists in the production process,the quality characteristic X obeys normal distribution N( µ， ),and according to the probabilistic properties of the normal distribution, there is That is,（ µ － 3 σ,µ＋ 3 σ ）is the practical value range of X. P｛µ － 3 σ ＜ Ｘ＜ µ＋ 3 σ ｝＝99.73 % 19 2
SmallprobabilityprincipleWhat are called small probability principle is thatsmall probability events usually couldn't happen.The criterion shows that if X obeys normaldistribution, the possibility that the possible value of Xis beyond control limit is only 0.27%.Therefore,it isgenerally believed that X won't exceed control limit.Of course,using small probability principle can alsoresult in error,but the possibility of making mistakes isexactlythe small probability2007.4Smallprobabilityprinciple20
Small probability principle 2007.4 Small probability principle Of course,using small probability principle can also result in error,but the possibility of making mistakes is exactly the small probability The criterion shows that if X obeys normal distribution, the possibility that the possible value of X is beyond control limit is only 0.27%.Therefore, it is generally believed that X won't exceed control limit. What are called small probability principle is that small probability events usually couldn't happen. 20