Presentation on theme: "Quality management: SPC II"— Presentation transcript:
1Quality management: SPC II Presented by:Dr. Husam Arman
2Histograms do not take into account changes over time. Control charts can tell us when a process changes
3Control Chart Applications Establish state of statistical controlMonitor a process and signal when it goes out of controlDetermine process capability
4Commonly Used Control Charts Variables datax-bar and R-chartsx-bar and s-chartsCharts for individuals (x-charts)Attribute dataFor “defectives” (p-chart, np-chart)For “defects” (c-chart, u-chart)
5Control chart functions Control charts are decision-making tools - they provide an economic basis for deciding whether to alter a process or leave it aloneControl charts are problem-solving tools - they provide a basis on which to formulate improvement actionsSPC exposes problems; it does not solve them!
6Control charts PROCESS Control charts are powerful aids to understanding the performance of a process over time.OutputInputPROCESSWhat’s causing variability?
7Control charts identify variation Chance causes - “common cause”inherent to the process or random and not controllableif only common cause present, the process is considered stable or “in control”Assignable causes - “special cause”variation due to outside influencesif present, the process is “out of control”
9Control charts help us learn more about processes Separate common and special causes of variationDetermine whether a process is in a state of statistical control or out-of-controlEstimate the process parameters (mean, variation) and assess the performance of a process or its capability
10Control charts to monitor processes To monitor output, we use a control chartwe check things like the mean, range, standard deviationTo monitor a process, we typically use two control chartsmean (or some other central tendency measure)variation (typically using range or standard deviation)
11Control chart components Centerlineshows where the process average is centered or the central tendency of the dataUpper control limit (UCL) and Lower control limit (LCL)describes the process spread
12Control chartControl charts are practical tools to monitor the evolution of production processes.In any production process a certain amount of natural variability will always exist (this is the cumulative effect of small and unavoidable causes)A process that is operating in the presence of chance causes of variation only is said to be in statistical control.
13Control chartA process that is operating in the presence of assignable causes (sources of variability that are not part of the chance causes) is said to be out of control.Three main sources of assignable causes:1) improperly adjusted or controlled machines (or failures);2) operator errors;3) defective raw materials.
14Control chart A control chart contains A center line (CL) An upper control limit (UCL)A lower control limit (LCL)(LCL)(UCL)(CL)Control limits are different from specification limits
15(LCL)(UCL)(CL)Basic criterionA point that plots within the control limits indicates that the process is in control → no action is necessaryA point that plots outside the control limits is evidence that the process is out of control→ Investigation and corrective action are required to find and eliminate assignable cause(s)There is a close connection between control charts and hypothesis testing (to test if the process is in a state of statistical control: we can fail to reject or reject this hypothesis)
18Control chartOne of the two main types of control charts is chart for VARIABLES (quality characteristics measured on a numerical scale; e.g. geometrical dimensions, weights, tensile strengths…)- (mean) control charts- R (range) control charts- s2 (sample variance) control charts- s (sample standard deviation) control charts- xi (control charts for individual measurements)
19Control chart for variables (Ch 5) Variables are the measurable characteristics of a product or service.Measurement data is taken and arrayed on charts.
20X-bar and R chartsThe X-bar chart - used to detect changes in the mean between subgroupstests central tendency or location effectsThe R chart - used to detect changes in variation within subgroupstests dispersion effects
21Step 1 Define the problem Use other quality tools to help determine the general problem that’s occurring and the process that’s suspected of causing it.brainstorm using cause and effect diagram, why-why, Pareto charts, etc.
22Step 2 Select a quality characteristic to be measured Identify a characteristic to study - for example, part length or any other variable affecting performancetypically choose characteristics which are creating quality problemspossible characteristics include: length, height, viscosity, temperature, velocity, weight, volume, density, etc.
23Step 3 Choose a subgroup size to be sampled Choose homogeneous subgroupsHomogeneous subgroups are produced under the same conditions, by the same machine, the same operator, the same mold, at approximately the same time.Try to maximize chance to detect differences between subgroups, while minimizing chance for difference with a group.
24Other guidelinesThe larger the subgroup size, the more sensitive the chart becomes to small variations in the process average.This increases data collection costs.Destructive testing may make large subgroup sizes infeasible.Subgroup sizes smaller than 4 aren’t representative of the distribution averages.Subgroups over 10 should use S chart.
25Step 4 Collect the dataRun the process untouched to gather initial data for control limits.Generally, collect subgroups (100 total samples) before calculating the control limits.Each time a subgroup of sample size n is taken, an average is calculated for the subgroup and plotted on the control chart.
26Step 5 Determine trial centerline for the Xbar chart The centerline should be the population mean, Since it is unknown, we use X double bar, or the grand average of the subgroup averages.
28Step 6 Determine trial control limits - Xbar chart The normal curve displays the distribution of the sample averages.A control chart is a time-dependent pictorial representation of a normal curve.Processes that are considered under control will have 99.73% of their graphed averages fall within six standard deviations.
30Determining an alternative value for the standard deviation (Xbar chart)
31Step 7 Determine trial control limits - R chart The range chart shows the spread or dispersion of the individual samples within the subgroup.If the product shows a wide spread, then the individuals within the subgroup are not similar to each other.Equal averages can be deceiving.Calculated similar to x-bar charts;Use D3 & D4
35R-bar chart exceptions Because range values cannot be negative, a value of 0 is given for the lower control limit of sample sizes of six or less (see D3 value in the previous table).
36Step 8 Examine the process - Interpret the charts A process is considered to be stable and in a state of control, or under control, when the performance of the process falls within the statistically calculated control limits and exhibits only chance, or common causes.
37Consequences of misinterpreting the process Blaming people for problems that they can’t controlSpending time and money looking for problems that do not existSpending time and money on unnecessary process adjustmentsTaking action where no action is warrantedAsking for worker-related improvements when process improvements are needed first
38Process variationWhen a system is subject to only chance causes of variation, 99.73% of the measurements will fall within 3 standard deviationsIf 1000 subgroups are measured, 997 will fall within the six sigma limits.
39Chart zonesBased on our knowledge of the normal curve, a control chart exhibits a state of control when:Two thirds of all points are near the center value.A few of the points are on or near the center valueThe points appear to float back and forth across the centerline.The points are balanced on both sides of the centerline.No points beyond the control limits.No patterns or trends.
40Identifying patternsSudden shift in the process averageCyclesTrends
45Step 9 Revise the chartsIn certain cases, control limits are revised because:out-of-control points were included in the calculation of the control limits.The process is in-control but the within subgroup variation significantly improves.
46Revising the charts Interpret the original charts Isolate the causes Take corrective actionRevise the chartOnly remove points for which you can determine an assignable cause
47Step 10 Achieve the purpose Our goal is to decrease the variation inherent in a process over time.As we improve the process, the spread of the data will continue to decrease.Quality improves!!
48Charts for Attributes Fraction nonconforming (p-chart) Fixed sample sizeVariable sample sizenp-chart for number nonconformingCharts for defectsc-chartu-chart
49Control Chart Selection Quality Characteristicvariableattributedefectivedefectnon>1?x and MRconstantsamplingunit?yesconstantsamplesize?yesp ornpnon>=10 ?x and Ryesnonoyesp-chart withvariable samplesizec ux and s
50Control Chart Design Issues Basis for samplingSample sizeFrequency of samplingLocation of control limits
51SPC Implementation Requirements Top management commitmentProject championInitial workable projectEmployee education and trainingAccurate measurement system