3 years ago

Six Sigma - Scientific Devices Australia

Six Sigma - Scientific Devices Australia

AN119 Dataforth

AN119 Dataforth Corporation Page 2 of 3Lean Sigma is a current industry trend in which aprocess is made lean through efficiencyimprovements before Six Sigma is applied to reducevariation.Six Sigma CalculationThe term “Six Sigma” comes from process capabilitystudies, which measure the extent to which aprocess meets customer requirements,specifications, or product tolerances. Sigmarepresents the standard deviation (variation) fromthe process mean of a statistical population. Basedon the calculation method used in process capabilitystudies, if a process has six standard deviationsbetween the process mean and the nearestspecification limit, almost no items will fail to meetspecifications. This is a Six Sigma process.The process capability index is computed using thefollowing formula, where C p = process capabilityindex, USL = upper specification limit, LSL = lowerspecification limit, and σ = sigma:C p = USL – LSL6 × σAs the equation indicates, higher C p values arefound in more capable processes. As the processstandard deviation goes up, or the mean of theprocess moves away from the center of thetolerance, fewer standard deviations will fit betweenthe mean and the nearest specification limit, therebydecreasing the sigma number and increasing thelikelihood of items outside specification.Numerous process sigma calculators are availableon the Internet that provide quick calculation of howa particular process is performing with regard to theSix Sigma goal. The calculation of a sigma level isbased on the number of defects per millionopportunities (DPMO). The formula to calculateDPMO is:DPMO = __ (Number of defects × 1,000,000)______((Number of opportunities/unit) × Number of units)If, for example, there are 38 defects, 10,000 units,and one defect opportunity/unit, the results wouldbe:DPMO 3,800Defects (%) 0.38Yield (%) 99.62Process sigma 4.17The process sigma calculation includes a 1.5 sigmashift to account for changes over time.The 1.5 Sigma ShiftSix Sigma Table1 690,0002 308,0003 66,8004 6,2105 3206 3.4This 1.5 sigma shift was added to the calculation asa result of years of data collection by Motorola,which showed that processes vary and drift overtime. Known as the Long-Term Dynamic MeanVariation, this shift is usually between 1.4 and 1.6sigma. The reporting convention of Six Sigma,however, requires process capability to be reportedin short-term sigma, which includes common causevariation but not special cause variation. As a result,short-term data typically indicates higher processcapability than long-term data. To determine longtermsigma, 1.5 sigma is subtracted from the shorttermsigma calculation.Therefore, a process that fits six sigma between theprocess mean and the nearest specification limit in ashort-term study will fit only 4.5 sigma over the longterm.In the book, Six Sigma Producibility Analysis andProcess Characterization, written by Mikel J. Harryand J. Ronald Lawson and published by Motorola in1992, there is a standard normal distribution tablethat is taken out to a z value of 6 (most such tablesonly go to a z value of about 3). Based on this table,six sigma translates to 2 defects per billionopportunities, while 3.4 defects per millionopportunities – commonly defined as Six Sigma –actually corresponds to a sigma value of 4.5. A bitconfusing, for sure, but the difference betweenshort-term and long-term process performanceexplains it, and incorporating the shift preventsunderestimating the defect levels that will probablybe found over time.Six Sigma BenefitsA definite focus on achieving measurable andquantifiable financial goals (cost reduction/profitincrease) with Six Sigma projects sets thismethodology apart from other quality improvementtactics. Financial benefits of potential processimprovement projects are used to help prioritize the

AN119 Dataforth Corporation Page 3 of 3projects; these benefits are then reassessed duringthe analyze phase of both DMAIC and DMADV andverified in the control phase of DMAIC and the verifyphase of DMADV. Closely linking Six Sigma projectsto a company’s bottom line gives everyone in theorganization a stake in the success of theseprojects. It also helps identify projects that involvecritical-to-quality aspects of the process and willprovide substantial process improvement.By the late 1990s, approximately two-thirds ofFortune 500 companies had started Six Sigmaprojects; as of 2006, Motorola had reported morethan $17 billion in savings as a result of Six Sigma.Dataforth has been involved with Six Sigma formany years and, in fact, the reliability of Dataforth’ssignal conditioners is better than Six Sigma.Dataforth Product Family13-Month Average DPMODSCA DIN Mount Signal Conditioners 0.60SCM5B Signal Conditioning Modules 0.01SCM7B Signal Conditioning Modules 0.12In addition to significant financial gain, Six Sigmaprovides organizations with the methodology andstructure to make decisions based on verifiable dataand statistical analysis and thereby to achievemeasurable quality improvements in manufacturingand business processes. Six Sigma projects aretruly a win-win situation as product quality is greatlyimproved while product defects and variation arereduced, employees are meaningfully involved in theoutcome of the projects, company profitability ismeasurably increased, and customer loyalty andsatisfaction are significantly enhanced.Dataforth ReferencesDataforth Corp. Website

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