Would you ever buy a product if you knew it has a low quality? Probably not. In addition to that, every quality issue is linked to cost so for example, scrap or rework. That's why another important focus in the area of lean is quality. Customer expectations need to be met, and cost for quality issues, for sure avoided. So lean aims for zero defects which means high quality for all products. Defects are prevented before they occur. According to the rule of ten, it costs ten times more to correct a defect at next downstream process from development to production, to customer use. That's why detecting defects only on the value chain prevents higher cost downstream. It is important to know. The aim of zero defects is to continuously improve processes to prevent the same mistakes from happening twice, and it is a basic rule. When you go to production, you need to make sure that people understand, okay, it happens once, but your next time, it should never happen again. Make sure that it won't. This requires, finding the root causes of failures to sustainability correct them. There two keying tools Poka Yoke and Six Sigma. Poka Yoke prevents human mistakes or makes errors immediately visible. It makes it impossible to use a product in a wrong way or avoid to transfer the product to the next production steps. One example for Poka Yoke is a shape of a SIM card. You know what it looks like, right? It's impossible for the user to place it wrongly into the phone. Just check it out. The USB port is another good example, since it makes it impossible for the users to plug a cable in the wrong direction. Poka Yoke can also prevent operators from making mistakes in the assembling process or in starting the machine. It reduces the amount of waste to use a tool to one. For example, some machines have two separate hand safety buttons to press at the same time to ensure both hands are on the machine. Another well known method is Six Sigma. Six Sigma is a method that enables the standardization of processes through statistical, monitoring and correction. Six Sigma comes from statistics, and is used, for example, for statistical process control. Based on Six Sigma, the capability of a process can be evaluated. For a specific product specification, like the size of a phone for example, a target value as well as acceptable upper and lower limits are defined. The target value is average of a normal distribution. The more variable the process the higher sigma, and there will be more products out of specification and therefore waste. For examples, phone screens that are too long to fit to the phone casing or that are too short to stay in place. In both cases it would be costly to repair these faults. The purpose of Six Sigma method is therefore to reduce Six Sigma so that the difference between the average plus the sigma and the average minus the sigma falls within the specifications. Just to put it into perspective, a Six Sigma process level means just 3.14 defects per million opportunities. Compare it with a surgical operation. A surgical operation process with a tolerant of the 4 sigma. It will prevent 500 failures per week. Can you imagine? These tools and approaches aim to increase quality. However, maximum quality is not always efficient and creates cost. In addition, it could be not appreciated by the customer. So we are to do with that. Sometimes the customer is not even able to see the difference any more. That's why lean tries to get rid of both, under quality but over quality. A useful tool to structure quota expectation is a CTQ diagram means critical-to-quality. It builds up top down like a logic tree. First, make a list of customer needs. Second, associate and expectation to each customer need and third, list the requirements to fulfill the expectations. They should be measurable. Indicators and tolerances must be then defined. In conclusion, this approach is a way to reach the right level of quality. Not too low, but more important, also not too high.
