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In the name of Allah,the Most Beneficient, Presented by Nudrat Rehman Roll# 07-47 www.bzupages.com • Chapter 3, Quality Improvement Tools • Total Quality Management • by Poornima M Charantimath www.bzupages.com WHAT IS A CONTROL CHART? A statistical tool used to distinguish between process variation resulting from common causes and variation resulting from special causes. www.bzupages.com Common causes SPECIAL CAUSES www.bzupages.com ELEMENTS OF CONTROL CHART Each point is usually a sample statistic (such as subgroup average) of the quality characteristic Center Line represents mean operating level of process LCL & UCL are vital guidelines for deciding when action should be taken in a process www.bzupages.com TWO KINDS OF VARIATIONS Natural Variations: Due to common causes, purely random, unidentified. Assignable Variations: Variability where causes can be identified. www.bzupages.com VARIATIONS www.bzupages.com Developing Control Charts 1. Prepare – Choose measurement – Determine how to collect data, sample size, and frequency of sampling – Set up an initial control chart 2. Collect Data – Record data – Calculate appropriate statistics – Plot statistics on chart www.bzupages.com 3. Determine trial control limits – Center line (process average) – Compute UCL, LCL 4. Analyze and interpret results – Determine if in control – Eliminate out-of-control points – Recomputed control limits as necessary www.bzupages.com 5. Use as a problem-solving tool – Continue to collect and plot data – Take corrective action when necessary 6. Compute process capability www.bzupages.com Out-Of-Control Process when a sample point (e.g., mean in an X-bar chart) falls outside the control lines, you have reason to believe that the process may no longer be in control. www.bzupages.com Types of Control Charts Control Charts for Variables • X-bar chart (mean chart) • R chart (range chart) • S chart (Sigma chart/Standard Deviation) Control Charts for Attributes • p chart • np chart • c chart • u chart www.bzupages.com Control Charts for Variables • X-bar chart: In this chart the sample means are plotted in order to control the mean value of a variable (e.g., size of piston rings, strength of materials, etc.). • R chart: In this chart, the sample ranges are plotted in order to control the variability of a variable. • S chart: In this chart, the sample standard deviations(a measure of the variation between individuals on a variable) are plotted in order to control the variability of a variable. www.bzupages.com Control Charts for Attributes • P chart: In this chart, we plot the percent of defectives (per batch, per day, per machine, etc.) • Np chart: In this chart, we plot the number of defectives (per batch, per day, per machine) as in the C chart. • C chart: In this chart , we plot the number of defectives (per batch, per day, per machine, per 100 feet of pipe, etc.),but assumes that defects of the quality attribute are rare. www.bzupages.com • U chart: In this chart we plot the rate of defectives, that is, the number of defectives divided by the number of units inspected (e.g., feet of pipe, number of batches). www.bzupages.com Advantages of attribute control charts • allow quick summaries of various aspects of the quality of a product • the engineer may simply classify products as acceptable or unacceptable, based on various quality criteria. Thus, attribute charts sometimes bypass the need for expensive, precise devices and time-consuming measurement procedures. • Also, this type of chart tends to be more easily understood by managers www.bzupages.com Advantages of variable control charts • Variable control charts are more sensitive than attribute control charts . Therefore, variable control charts may alert us to quality problems before any actual "unacceptables" (as detected by the attribute chart) will occur. www.bzupages.com Control Chart Applications • Establish state of statistical control • Monitor a process and signal when it goes out of control • Determine process capability www.bzupages.com SELECTION OF CONTROL CHART Data Variable Data n = 2 to 9 n = 10 or more X bar Range Chart X bar Sigma Chart Attribute Data n=1 Defectives/ Units Defects non-normal data normal data n fixed n varies n fixed n varies Run Chart Individual X Range Chart np Chart p Chart c Chart u Chart www.bzupages.com QUESTIONS www.bzupages.com