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## ENGM 720 - Lecture 09

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**ENGM 720 - Lecture 09**Control Charts for Individuals ENGM 720: Statistical Process Control**Review**• Shewhart Control charts • Are for sample data from an approximate Normal distribution • Three lines appear on all Shewhart Control Charts • UCL, CL, LCL • Two charts are used: • X-bar for testing for change in location • R or s-chart for testing for change in spread • We check the charts using 4 Western Electric rules • Attributes Control charts • Are for Discrete distribution data • Use p- and np-charts for tracking defective units • Use c- and u-charts for tracking defects on units • Use p- and u-charts for variable sample sizes • Use np- and c-charts with constant sample sizes ENGM 720: Statistical Process Control**Individual Measurements**• Sometimes repeated measures in a subsample don't make sense: • inventory level • accounts payable – price of an item • Other reasons for using individual measurements • Variation in sample only reflects measurement error, e.g., batch production of chemicals • Automated inspection – every unit is analyzed • Production rate very slow – inconvenient to wait for large enough sample ENGM 720: Statistical Process Control**Control Charts for Individual Measurements**• Notes about Individuals Charts: • Sample points must be relatively frequent • There is more sampling error (false alarm & insensitivity) • Sample points tend to be non-normal • points are not averages and central limit theorem does not apply • Control Chart Types for Individuals: • Shewhart x-chart and Moving Range chart (2 charts!) • MA – Moving Average chart • EWMA – Exponentially Weighted Moving Average chart • CUSUM – Cumulative Sum chart ENGM 720: Statistical Process Control**Moving Range Control Chart**• MR2i = |xi – xi-1| - or - MR3i = |xi – xi-2| • Computation of the Moving Range: ENGM 720: Statistical Process Control**Moving Range Control Chart, Cont'd**• General model for moving range chart: • Plot MR2i = |xi – xi-1| or MR3i = |xi – xi-2| on control chart • Substituting estimates for mR and sR and using “3-sigma” limits: • Where MR is: ENGM 720: Statistical Process Control**Moving Range Control Chart, Cont'd**• For MR2 use table factors d3 and d2 for n = 2 For MR3 use table factors d3 and d2 for n = 3 • Very similar to Range chart, except we’re using moving range instead of average range ENGM 720: Statistical Process Control**x Control Chart(Individual Measurements Chart)**• Plot sample statistic: x • General model for x chart • Substituting estimates for mx and sx and using 3-sigma limits where: ENGM 720: Statistical Process Control**x Control ChartCont'd**• x chart upper and lower limits: ENGM 720: Statistical Process Control**Cautions for x & Moving Range charts:**• Always check x’s for normality • If x’s not normal, control limits are inappropriate • Use run/zone rules ONLY if the x’s are Normal • Very BAD at detecting small shifts, i.e., shifts < 2s ENGM 720: Statistical Process Control**Montgomery (5th ed.) Example 5-5, p. 250Viscosity of**aircraft primer paint • Plot MR2i = |xi – xi-1| on control chart • From initial data compute x and MR: (since d2 = 1.128 for n =2) • Control Limits for Moving Range chart (use D4 & D3 for n =2) ENGM 720: Statistical Process Control**Ex.: Viscosity of aircraft primer paint, Cont'd**• Computed control Limits for x Chart ENGM 720: Statistical Process Control**Alternatives to Shewhart Control Charts**• All control charts so far have been Shewhart-type Charts • use information about the process contained in the last plotted point • ignore information given by the entire sequence of points, unless sensitizing rules are used • Shewhart charts are relatively insensitive to small shifts, ex. shifts < 1.5s • Three Alternative charts: • MA – Moving Average control chart • EWMA – Exponentially Weighted Moving Average control chart • CUSUM – Cumulative-sum control chart ENGM 720: Statistical Process Control**MA Control Chart(Non-Shewhart Control Chart)**• Plot sample statistic: average of last w data points (Mi ) • Computing point to plot ( Mi ) for the chart: • Estimate for μ(to find center line): • Estimate for sto find control limits, (NOTE: limits change with each point): ENGM 720: Statistical Process Control**MA Control Chart(Non-Shewhart Control Chart)**• General model for MA control chart • Notes: • Picking w larger makes chart faster to detect to smaller shifts • Picking w smaller makes chart more sensitive to larger shifts • MA is better at detecting smaller shifts than a Shewhart chart, but not as effective as a EWMA or CUSUM chart ENGM 720: Statistical Process Control**EWMA - Exponentially Weighted Moving Average Control Chart**• The EWMA control chart is good for detecting small shifts • EWMA can be used to monitor • process mean or variance • Plot sample statistic: • zi = l (current x) + (1 - l)(weighted avg of past x's) • That is: zi = lxi + (1- l)zi -1 • Use estimate for OR target value • l is weighting factor, where 0 < l < 1 ENGM 720: Statistical Process Control**Example Computing EWMA Statistic**• Process mean is 14.31. Here are the first three observations. • Compute the EWMA statistic, zi, with weight l = 0.2. z1 = (0.2)(14.56) +(0.8)(14.31) = 14.36 z2 = (0.2)(13.88) +(0.8)(14.36) = 14.26 z2 = (0.2)(13.98) +(0.8)(14.26) = 14.20 ENGM 720: Statistical Process Control**EWMA Control Limits**• Standards Given: • Standards not given - use estimates for mz and sz: • Notice: σ depends on the observation number i • Use to estimate • Typical values for l and L: • 0.05 l 0.25 and 2.6 L 3.054 ENGM 720: Statistical Process Control**Example 9-2, p. 435: EWMA Chart for Process Mean**• Set up EWMA chart for following data from a process with mean 10 and std dev 1. Use l = 0.1 and L = 2.7. Z1=9.945 UCL=10.27 LCL=9.73 Z2=9.799 UCL=10.36 LCL=9.63 Z3=9.929 UCL=10.42 LCL=9.58 Z30=10.052 UCL=10.62 LCL=9.38 ENGM 720: Statistical Process Control**Example: EWMA Chart Cont'd**• If there is a point above Process is out-of-control ENGM 720: Statistical Process Control**Design of EWMA Control Chart:How to pick l and L**• Use smaller l to detect smaller shifts • Usual choices: l = 0.05, l = 0.10, l = 0.20 • Reasonable configuration: • For l = 0.20 let L = 3 • For smaller l, use slightly smaller L • For l = 0.05 let L 2.6 • For l = 0.10 let L 2.8 • See Table 9-11, p.437 for ARL’s ENGM 720: Statistical Process Control**CUSUM Control Chart**• Incorporates all the information in the sequence of sample values by • plotting the cumulative sums of the deviations of the sample values from a target value, m0 • CUSUM can be used to monitor • process mean • defectives • defects • variance • CUSUM can have sample size n 1 • We concentrate on sample size n = 1 ENGM 720: Statistical Process Control**Basic Principle of CUSUM**• Plot Ci – CUSUM sample statistic • Example: Say target m0 = 10 • If the process remains in-control, Ci remains near 0 ENGM 720: Statistical Process Control**Tabular CUSUM Control Chart**• xi ~ N(m0, s) - quality characteristic • CUSUM works by compiling the statistics: • Ci+ = accumulated deviations above m0 (resets to 0 if it would go negative) • Ci– = accumulated deviations below m0(resets to 0 if it would go negative) • The Tabular CUSUM • Record following values in table: • where starting values are ENGM 720: Statistical Process Control**Tabular CUSUM Control Chart Cont'd**• Let m1 = out-of-control value then • K is reference value chosen halfway between target m0 and out-of-control value • With shift expressed in std dev units, i.e., • and accumulate deviations from μ0 that are greater than K • and are reset to zero upon becoming negative ENGM 720: Statistical Process Control**How to Determine if Process Out-of-Control?**• H - decision interval • If or exceed the decision interval (H), the process is considered out-of-control • Rule of thumb value for H • Choose H to be five times the process standard deviation, H = 5s • Counters N+ and N– record the number of consecutive periods the CUSUM and rose above zero, respectively. • The counters can be used to indicate when the shift most likely occurred ENGM 720: Statistical Process Control**Example 9-1, p. 418**• m0 =10, n =1, s = 1.0 • Say magnitude of shift we want to detect is ds = 1 (1.0) = 1.0 • Using the Rule of thumb for H … • So … • and … • also … ENGM 720: Statistical Process Control**Tabular CUSUM Example**ENGM 720: Statistical Process Control**Notes about CUSUM control charts**• Do not apply run/zone rules • Successive values of and are not independent ENGM 720: Statistical Process Control**Recommendations for CUSUM design**• Let H = hs and K = ks where s is process std dev • Using h = 4 or h =5 and k = 1/2 gives CUSUM w/ good ARL ENGM 720: Statistical Process Control**Guidelines for Implementing Control Charts**• Determine which process or product characteristic(s) to control • Determine where the charts should be implemented in process • Choose proper type of control charts • Decide what actions should be taken to improve processes • Select data-collection systems and computer software ENGM 720: Statistical Process Control**Determine Which Characteristic to Control and Where to Put**Charts • To start, apply charts to any process or product characteristics believed important. • Charts found unnecessary are removed; others that may be required are added. (Usually more charts to start than after process has stabilized) • Keep current records of all charts in use, i.e., types and parameters of each. • If charts used effectively number of charts for variables increases and number of attributes charts decreases ENGM 720: Statistical Process Control**Determine Which Characteristic to Control and Where to Put**Charts • At beginning, use more attributes charts applied to finished units, i.e., near end of process. As more is learned about the process, these are replaced with variables charts earlier in process on critical process characteristics that affect nonconformities.Rule of thumb: the earlier in the process that control can be established, the better. • Control charts are an on-line process monitoring procedure; Maintain charts as close to work center as possible.Operators and process engineers should be directly responsible for using, maintaining and interpreting charts ENGM 720: Statistical Process Control**Choosing Proper Type of Control Chart: Variables Charts**• Use (x & R) or (x & S) charts when: • New process or product coming online • Chronically troubled process • Wish to reduce downstream acceptance sampling • Using attributes charts but yield still unacceptable • Very tight specifications • Operator decides whether or not to adjust process • Change in product specs desired • Process capability (stability) must be continually demonstrated ENGM 720: Statistical Process Control**Choosing Proper Type of Control Chart: Attributes Charts**• Use p, np, c or u charts when: • Operators control assignable causes and it is necessary to reduce fallout • Process is complex assembly operation and product quality measured in terms of occurrence of nonconformities: e.g. computers, automobiles • Measurement data cannot be obtained • Historical summary of process performance is necessary. Attributes charts are effective for summarizing a process for management ENGM 720: Statistical Process Control**Choosing Proper Type of Control Chart: Individuals Charts**• Use (x & MR), MA, EWMA, or CUSUM charts when: • Repeated measures do not make sense • Inconvenient / impossible to obtain more than one measurement per sample • Automated testing allows you to measure every unit (EWMA chart may be best) • Data becomes available very slowly and waiting for a larger sample is impractical. ENGM 720: Statistical Process Control**Assignment:**• Reading: • Finish Chapter 7 • Sections 7.1 and 7.2 throughp.313 • Sections 7.3 through p.325 • Sections 7.3.2 and 7.5 • Start Chapter 9 • 9.1 through 9.1.3 • 9.2 through 9.2.4 • 9.3 • Start Chapter 6 • 6.4 though p. 272 • Assignments: • Obtain the Chart Factors table from Materials Page • Use the data on the Excel sheets to do the charting • verify the control limits by hand calculations • Access Excel Template for P, NP, C, & U Control Charts: • Download Assignment 6 for practice • Access Excel Template for Individuals Control Charts • Download Assignment 7 for practice ENGM 720: Statistical Process Control**Questions & Issues**ENGM 720: Statistical Process Control