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  1. 1. Statistical Quality Control A Presentation BY Aman Wadhawan 89  Namish Mishra 86  Rahul Chowdhry 67  Mohit Singh  88 Akshay Karnatak 70
  2. 2. Meaning Of Statistical Quality Control Statistical quality control can be simply defined as an economic & effective system of maintaining & improving the quality of outputs throughout the whole operating process of specification, production & inspection based on continuous testing with random samples
  3. 3. Categories Of SQC Descriptive statistics Statistical Process Control Acceptance Sampling
  4. 4. Causes Of Variation In Quality Common Causes Of Variation Assignable Causes Of Variation
  5. 5. Method Of SQC Process Control It extend the use of descriptive statistics to monitor the quality of the product and process  Under this the quality of the products is controlled while the products are in the process of production.  It is secured with the technique of control charts
  6. 6. PURPOSE & USES OF CONTROL CHARTS  A control chart (also called process chart or quality control chart) is a graph that shows whether a sample of data falls within the common or normal range of variation.  Helps in determining the quality standard of the products.  Helps in detecting the chance & assignable variations in the quality standards by setting two control limits  Indicates whether the production process is in control or not.  Ensures less inspection cost & time in the process control.
  7. 7. Types Of Control Charts  Control Chart For Variables X-Chart R-Chart  Control Chart For Attributes P-Chart NP-Chart C-Chart
  8. 8. X-Chart  A mean control chart is often referred to as an x-bar chart. It is used to monitor changes in the mean of a process.  To construct a mean chart we first need to construct the center line of the chart.
  9. 9.  To construct the upper and lower control limits of the chart, we use the following formulas.  Upper control limit(UCL) =  Lower control limit (LCL) =  Where, = the average sample means  Z = standard normal variable (2 for 95.44% confidence, 3 for 99.74%  = standard deviation of the distributed sample means, computed as  = population (process) standard deviation  N = sample size confidence)
  10. 10. Question A quality control inspector at the Cocoa Fizz soft drink company has taken 5 samples with four observations each of the volume of bottles filled. The data and the computed means are shown in the table. If the standard deviation of the bottling operation is 0.14 ounces, use this information to develop control limits of three standard deviations for the bottling operation.
  11. 11. Solution = 15.95 The control limits are UCL = LCL =
  12. 12. RANGE - CHART  Range (R) charts are another type of control chart for variables.  Whereas x-bar charts measure shift in the central tendency of the process, range charts monitor the dispersion or variability of the process.  The method for developing and using Rcharts is the same as that for x-bar charts. The center line of the control chart is the average range, and the upper and lower control limits are computed as follows.
  13. 13.  CL = R  UCL = D4. R  LCL = D3. R  Where as , CL =central line  UCL = upper control limit  LCL = lower control limit  D3 and D4 are factors of r chart
  14. 14. Using Mean and Range Charts Together  You can see that mean and range charts are used to monitor different variables. The mean or x-bar chart measures the central tendency of the process, whereas the range chart measures the dispersion or variance of the process. Since both variables are important, it makes sense to monitor a process using both mean.
  15. 15. C-CHART  A control chart used to monitor the number of defects per unit.  Examples are the number of returned meals in a restaurant, the number of trucks that exceed their weight limit in a month, and the number of bacteria in a milliliter of water.
  16. 16. WHEN TO USE C-CHART? Use C-Charts for discrete defects when there can be more than one defect per unit Number of flaws or stains in a carpet sample cut from a production run Number of complaints per customer at a hotel
  17. 17.  Note that the types of units of measurement we are considering are a period of time, a surface area, or a volume of liquid.  The average number of defects, is the center line of the control chart. The upper and lower control limits are computed as follows: UCL c c z c LCL c c z c
  20. 20. NP Chart • Sometimes it is necessary (or convenient) to look at the number of defective items rather than the proportion of defective items. • we use np - charts instead of p – charts • Difference between p - charts and np - charts is that in the later case y-axis represents the number of defective items in a subgroup
  21. 21. Example Serial No. Sample Size No Of Defective Pens 1 100 2 2 121 2 3 81 0 4 100 1 5 121 2 Total 523 7
  22. 22. Solution  CL= np- bar  Np-bar= total no of defects/ total no inspected samples  Np-bar= 7/5= 1.4  LCL = np − 3√np(1 − p),  1.4- 3√1.4(1-0.0133)=-2.13  UCL= np+3√np(1-p)  UCL= 4.93  P= proportion of defect/total units
  23. 23. SQC in Services  Service Organizations have lagged behind manufacturers in the use of statistical quality control  Statistical measurements are required and it is more difficult to measure the quality of a service    Services produce more intangible products Perceptions of quality are highly subjective A way to deal with service quality is to devise quantifiable measurements of the service element  Check-in time at a hotel  Number of complaints received per month at a restaurant  Number of telephone rings before a call is answered  Acceptable control limits can be developed and charted © Wiley 2007
  24. 24. SQC Across the Organization  SQC requires input from other organizational functions, influences their success, and are actually used in designing and evaluating their tasks     © Wiley 2007 Marketing – provides information on current and future quality standards Finance – responsible for placing financial values on SQC efforts Human resources – the role of workers change with SQC implementation. Requires workers with right skills Information systems – makes SQC information accessible for all.