Quality is the relative term which mean that something is high or low quality compare to what its require to be and according to international organization for standardization (ISO), Quality is the degree to which a product or services are fulfill requirements and provide for value its prices. For most people quality is termed good vale getting your money's' worst. Quality is the degree to which a product or services are fulfill requirements and provide for value its prices. Statistics is the mathematical interpretation of numerical data and several statistical term are used in quality control. Control limit are the boundary of acceptable variations. If random factor cause variation, they will not tend to cancel each other out the central limit theorem. The central point in the distribution is the mean which is represented by Greek letter mu and if you choose interval called bin and count the number of sample that fall into each interval , the result is the frequency distribution. If you chart the distribution and the factors that cause the variation are random ,the frequency distribution is the normal distribution which look like bell shape. The center of the normal distribution is called mean and the average variation is calculated in a special ways that find the average of the square of the differences between sample and the mean and ten takes the square root . This average difference of the square is called the standard deviation which is represented by the Greek letter sigma. About 68 percent of the sample are within one standard deviation, 95.4 percent are within two, and 99.7 percent are within three distribution.