We are at lecture 35. Now, this lecture is on process capability studies. The topic we are going to cover in this lecture includes the capability metrics such as CP CPK and CPM as well as assessing the process capability. capability of the process can be found once we know standard deviation sigma of the process and specification limit given by the customer two commonly used process capability indices are potential process capability, CP process capability index CPK capability of a process can be measured in two steps. Measure the variability of a process, calculate the control limits and compare it with the specification with process capability CP is calculated as specification width divided by process width. In other words, the upper specification limit u s l minus the lower specification limit ls l divided by upper control limit UCL minus lower control limit l see L. We know that you see l is equal to mu plus three sigma and LC l is equal to mu minus three sigma.

Hence, UCL, minus LCL is equal to three sigma minus, minus three sigma which comes as Six Sigma process capability of the processes is equal to specification width divided by six sigma. let us calculate the process capability of the suppliers that we have discussed in the previous lecture. The specification width of customer requirements is 45 minus 35 equals to 10 mm. Now, for supplier a, the process standard deviation is equals to 3.76. So, six multiplied by standard deviation comes as 25.58 the process capability index for supplier A is equal to 10 divided by 22.58 which comes as 0.44. Similarly, we can calculate the process capability for all other suppliers.

Now, by seeing the capability What can you say? Which supplier Are you going to select? Definitely supply a D and C but not a and b. Now, the most Important point, the standard deviation is the only variable here pressed all our constants and capability of any process, we only depends upon the process standard deviation in order to increase your capability, reduce your standard deviation. In other words, reduce variation. We have just discussed one measure for capability, that is, capability index or CP.

CP as one major weakness, as in real life hardly any process remains centered to target. In such cases, process capability index CPK is better measure to analyze the process CPK is similar to CP here it is calculated as upper specification minus process mean divided by three sigma or process mean minus Louis specification divided by three sigma whichever is less will be the process capability let us understand with an example. We got mean of One process as 22 and standard deviation as 0.8 u s L is equals to 24 and ls l is equals to 18 that is find out the CPK calculation for CP is shown on the screen which comes out as 1.25 the calculation of CPK is shown now, we got two CPK values 0.83 and 1.67. Now, we have to take the minimum CPK as the capability of process that is 0.83 Have you seen the difference?

We got CP as 1.25. But the real capability of the process is less than 1.25 it is only 0.83 by finding CPK we can also predict about the center of the process. If CP and CPK are same, it means the process is centered since the beginning I've been using water Six Sigma capability of processes that is understand what does that mean? Let us review the processes operating in different sigma level and the defects associated with that sigma level first of all a process operating in three sigma level identify the mean and standard deviation of a process to find out process control limits that is mean plus minus three sigma. Now, find out where does the customer specification lies if it lies on the control limit have three sigma level then the process is said to be operating at three sigma level to improve the process, we need to reduce sigma a process operating in three sigma level produces high percentage of defects that will be 66,800 defects and 1 million opportunities.

Let us analyze, process operating and four sigma level identify the standard deviation and its control limit for any process Then find out where does the customer specification lies if the specification limits lies at four standard deviation from mean then the process is said to be operating at four sigma level a process operating in four sigma level producers fuel percentages of defects approximately six to one zero defects in 1 million opportunities. Similarly, process operating in Six Sigma capability identify the control limits and specification limits now, find out Where does the customer specification lies? If it lies on six standard deviation from mean then the process is said to be operating at Six Sigma level. There are very low chances of making defects, approximately 3.4 defects in 1 million opportunities. process capability studies are easy with Minitab. Let us learn to calculate the process capability with the help of Minitab those who want to practice can download and double click on the file process capability files consist of post set of quality data of post suppliers respectively.

I will show how to do capability analysis for supplier a you may repeat the procedure for other suppliers also click Start quality tools, capability analysis normal the capability analysis dialog box will open type supplier a at the box named single column in the box named subgroup size, type, one time type, lower specification as 35 and above specification as 45. Click OK to review the graphic window of capability analysis. Could you relate this graph with one of our previous lectures? What is it called? Yes, this is the same that we had discussed earlier the histogram I had mentioned during the lecture of histogram that we are going to see histograms at many places like this. Well, at the left hand side of the graph, are you able to see all those parameters of our interest the mean standard deviation on the right hand side, could you see the capability indices both CP and CPK.

Now, let us analyze whether this process is capable process is set to be capable only when its CP is more than one what a CP and CPK here both are less than one that means, this process is not capable. Now, whatever crossing upper and lower specification are the defects you can also see that there are almost 160 thousand defects parts per million opportunities. That's all for this lecture. Let us move to the next lecture, which is further extension of this lecture. Thank you