Statistical Process Control

Overview Variation Control charts – R charts – X-bar charts – P charts

Measures performance of a process Primary tool - statistics Involves collecting, organizing, & interpreting data Used to: – Control the process as products are produced – Inspect samples of finished products Statistical Quality Control (SPC)

Bottling Company Machine automatically fills a 20 oz bottle. Problem with filling too much? Problems with filling to little? So Monday the average is 20.2 ounces. Tuesday the average is 19.6 ounces. Is this normal? Do we need to be concerned? Wed is 19.4 ounces.

Natural Variation Machine can not fill every bottle exactly the same amount – close but not exactly.

Assignable variation A cause for part of the variation

SPC Objective: provide statistical signal when assignable causes of variation are present

Control Charts R Chart Variables Charts Attributes Charts X Chart P C Continuous Numerical Data Categorical or Discrete Numerical Data Control Chart Types

Characteristics for which you focus on defects Classify products as either ‘good’ or ‘bad’, or count # defects – e.g., radio works or not Categorical or discrete random variables Attributes Measuring quality Characteristics that you measure, e.g., weight, length May be in whole or in fractional numbers Continuous random variables Variables

Show changes in data pattern – e.g., trends Make corrections before process is out of control Show causes of changes in data – Assignable causes Data outside control limits or trend in data – Natural causes Random variations around average Control Chart Purposes

Figure S6.7

Steps to Follow When Using Control Charts TO SET CONTROL CHART LIMITS 1.Collect samples of n=4 or n=5 a stable process compute the mean of each sample. 2.Calculate control limits Compute the overall means Calculate the upper and lower control limits.

Steps to Follow When Using Control Charts - continued TO MONITOR PROCESS USING THE CONTROL CHARTS: 1.Collect and graph data Graph the sample means and ranges on their respective control charts Determine whether they fall outside the acceptable limits. 2.Investigate points or patterns that indicate the process is out of control. Assign causes for the variations. 3.Collect additional samples and revalidate the control limits.

Control Charts for Variables Glacier Bottling Management at Glacier Bottling is concerned about their filling process. In particular, they want to know whether or not the machines are really filling the bottles with 16 ounces. Create an Xbar chart that will be used to monitor the process. Collected data for 25 days. Each day, pulled 4 bottles from the filling line and measured the amount in the bottle.

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs

Glacier Bottling Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs Remember: There are 25 samples of size 4 to calculate the control limits. We are doing the first 5 right now…

Setting Control Limits for R chart

Monitors variability in process Variables control chart – Interval or ratio scaled numerical data Shows sample ranges over time – Difference between smallest & largest values in inspection sample R Chart

Sample Range at Time i # Samples From Table S6.1 R Chart Control Limits

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs 4R Glacier Bottling – 15.83=0.19

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs 4R Glacier Bottling – 15.83= – 15.85=0.27

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs 4R Rbar =0.29 ounce

Glacier Bottling R -Charts UCL R = D 4 R LCL R = D 3 R R = 0.29

Control Chart Factors Control Chart Factors Factor for UCLFactor forFactor Size ofand LCL forLCL forUCL for Samplex-ChartsR-ChartsR-Charts (n)(A 2 )(D 3 )(D 4 ) This chart is in your text and will be provided for exams if needed.

Glacier Bottling R -Charts UCL R = D 4 R LCL R = D 3 R R = 0.29 D 4 = D 3 = 0 UCL R = (0.29) = ounce LCL R = 0(0.29) = 0 ounce

Glacier Bottling R -Charts UCL R = D 4 R LCL R = D 3 R R = 0.29 D 4 = D 3 = 0 UCL R = (0.29) = ounce LCL R = 0(0.29) = 0 ounce

SETUP CHARTS Glacier Bottling

MONITORING Glacier Bottling

Figure S6.7

Glacier Bottling

Figure S6.7

Glacier Bottling

Figure S6.7

Glacier Bottling

Figure S6.7

Glacier Bottling

Figure S6.7

Setting Control Limits for Xbar chart

Monitors process average Variables control chart – Interval or ratio scaled numerical data Shows sample means over time  X Chart

 X Chart Control Limits Sample Range at Time i # Samples Sample Mean at Time i From Table S6.1

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs 4RXbar Glacier Bottling ( )/4 =

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs 4RXbar Glacier Bottling ( )/4 =

Bottle Volume in Ounces Sample NumObs 1Obs 2Obs 3Obs 4RXbar RBar = 0.29 ounce XBarBar = ounces

Xbar –Chart Glacier Bottling: Setting Control Limits for XBar chart UCL x = x + A 2 R LCL x = x - A 2 R Rbar = 0.29 xbarbar =

Control Chart Factors Control Chart Factors Factor for UCLFactor forFactor Size ofand LCL forLCL forUCL for Samplex-ChartsR-ChartsR-Charts (n)(A 2 )(D 3 )(D 4 ) This chart is in your text and will be provided for exams if needed.

X –Chart Glacier Bottling: Setting Control Limits for XBar chart UCL x = x + A 2 R LCL x = x - A 2 R R = 0.29 x = A 2 = = = = UCL x = (0.29) = oz.

X –Chart Glacier Bottling: Setting Control Limits for XBar chart UCL x = x + A 2 R LCL x = x - A 2 R R = 0.29 x = A 2 = = = = UCL x = (0.29) = oz. LCL x = – (0.29) = oz.

Glacier Bottling

Monitoring Process with R chart and Xbar chart

Figure S6.7 Your Turn: Buzz Group Monitoring the bottling process (3 pages)

Shows % of nonconforming items Attributes control chart – Nominally scaled categorical data e.g., good-bad p Chart

p Chart Control Limits # Defective Items in Sample i Size of sample i z = 2 for 95.5% limits; z = 3 for 99.7% limits

HOMETOWN BANK Hometown Bank The operations manager of the booking services department of Hometown Bank is concerned about the number of wrong customer account numbers recorded by Hometown personnel. Each week a random sample of 2,500 deposits is taken, and the number of incorrect account numbers is recorded. The records for the past 12 weeks are shown in the following table. Is the process out of control? Use 3-sigma control limits.

Hometown Bank UCL p = p + z  p LCL p = p - z  p  p = p (1 - p )/ n SampleWrong NumberAccount Number Total 147 Total defectives Total observations p = n = 2500 Control Charts for Attributes

Control Charts for Attributes Hometown Bank UCL p = p + z  p LCL p = p - z  p  p = p (1 - p )/ n SampleWrong NumberAccount Number Total (2500) p = n = 2500

Control Charts for Attributes Hometown Bank UCL p = p + z  p LCL p = p - z  p  p = p (1 - p )/ n SampleWrong NumberAccount Number Total 147 p = n = 2500

Control Charts for Attributes Hometown Bank UCL p = p + z  p LCL p = p – z  p  p = p (1 – p )/ n n = 2500 p =

Control Charts for Attributes Hometown Bank UCL p = p + z  p LCL p = p – z  p  p = (1 – )/2500 n = 2500 p =

Control Charts for Attributes Hometown Bank UCL p = p + z  p LCL p = p – z  p  p = n = 2500 p =

Control Charts for Attributes Hometown Bank  p = n = 2500 p = UCL p = (0.0014) LCL p = – 3(0.0014)

Control Charts for Attributes Hometown Bank  p = n = 2500 p = UCL p = (0.0014) LCL p = – 3(0.0014) Why 3? 3-sigma limits Also to within 99.7%

UCL p = LCL p = Control Charts for Attributes Hometown Bank  p = n = 2500 p =

p -Chart Wrong Account Numbers

p -Chart Wrong Account Numbers

p -Chart Wrong Account Numbers Investigate Cause

Figure S6.7

Which control chart is appropriate? Webster Chemical Company produces mastics and caulking for the construction industry. The product is blended in large mixers and then pumped into tubes and capped. Webster is concerned whether the filling process for tubes of caulking is in statistical control. The process should be centered on 8 ounces per tube. Several samples of eight tubes are taken and each tube is weighed in ounces.

Which control chart is appropriate? Webster Chemical Company produces mastics and caulking for the construction industry. The product is blended in large mixers and then pumped into tubes and capped. Webster is concerned whether the filling process for tubes of caulking is in statistical control. The process should be centered on 8 ounces per tube. Several samples of eight tubes are taken and each tube is weighed in ounces. X-bar and R charts

Which control chart is appropriate? A sticky scale brings Webster’s attention to whether caulking tubes are being properly capped. If a significant proportion of the tubes aren’t being sealed, Webster is placing their customers in a messy situation. Tubes are packaged in large boxes of 144. Several boxes are inspected. The number of leaking tubes in each box is recorded.

Which control chart is appropriate? A sticky scale brings Webster’s attention to whether caulking tubes are being properly capped. If a significant proportion of the tubes aren’t being sealed, Webster is placing their customers in a messy situation. Tubes are packaged in large boxes of 144. Several boxes are inspected. The number of leaking tubes in each box is recorded. P charts

Figure S6.7 Your Turn: Buzz Group Are these samples in control? (1 page)

How to do SPC Charts In Excel