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- Index
Groups
Groups are categories that the data can be divided into. Examples are: operator producing or testing the unit, line the unit was manufactured on and the cavity that the unit was made in. When the values fall into to such groups there is the possibility that the groups are different. If each group fits the normal distribution, but if some groups are shifted left or right relative to other groups, the resulting histogram can appear to have multiple peaks (modes). Such data is called bi-modal or multi-modal. As a result, it is likely to fail a normality test.
Such multi-modal data may pass the Skewness-Kurtosis Specific test and you may be able to proceed with a variables sampling plan or normal tolerance interval. However, it may sometimes be necessary to divide the data into the groups and test each group separately. For example, in validating an injection molding process, the die used may have 4 cavities for forming 4 parts at a time. If the cavities are different, it may be necessary to validate each cavity separately. Before the data is split into subgroups, there should be statistical evidence the groups are different. The group each value is from should be entered in the Group column of the Data window. Then a comparison of the groups is automatically performed and displayed in Tab 6 of the Test Distribution window.
The difference between groups and ordered data is that groups are categories that are unordered whereas ordered data consists of subgroups that are ordered relative to time. Both can produce multi-modal data.