In this case the number of nonconformities is divided by the length of the roll in hundreds of sheets. The c chart is appropriate only when the statistic of interest is the number of occurrences of an attribute in a given area of opportunity, where the area of opportunity remains constant for all subgroups. Here, the area of opportunity (subgroup size) varies because one roll constitutes a sample and the rolls are of varying length. In this case, it is the number of nonconformities divided by the length of the roll in hundreds of sheets. is a non-probabilistic sample because the sampler has no prior knowledge of the stratification, has no equally spaced distribution plan, and likely has skewed the data by taking the sample from the top, but apparently not from the very bottom.Ĭorrect! The u chart is appropriate when the statistic of interest is the ratio of the number of occurrences of an attribute to the area of opportunity. Therefore, the analyses of the initial product will not represent the received product, regardless how careful and accurately the analyses are done by the supplier.ĭrawing a convenience sample from multiple points that can be easily accessed is a non-probabilistic sample.ĭrawing a quota sample with one sample off the top and from five more standardized depths in the railcar. The product will have changed during transit in terms of moisture content or in other properties such as changes from particles blown off the car during shipment. Drawing a slipstream sample by taking a representative slice of the bulk material as the car is being unloaded being certain to capture all of the generated dust samples the entire railcar following arrival and allows assessment of moisture and changes in material from initial shipment.