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normal curvea frequency distribution which has a bell-shaped symmetrical pattern where it is possible to calculate the height of the curve (the frequency) corresponding to any given numerical value.
Normal curves are used in statistical QUALITY CONTROL to establish the limits of variation in the size or other characteristic of a product which is to be regarded as acceptable in SAMPLES of a product being tested. For example, if a component is intended to measure an average of 100 mm across and the standard deviation is 2 mm, then the testers would expect 95% of a sample of components to measure between 96 mm and 104 mm across, and 99% of the sample to measure between 94 mm and 106 mm across. If the latter range of sizes was felt be acceptable in terms of the technical performance of the product, then this confidence interval would lead to the whole batch of components being rejected if more than 1% of sample components were larger or smaller than these broad limits. By contrast, if a tougher criterion was imposed and only components measuring 96 mm to 104 mm across were to be regarded as technically acceptable, then 95% of sample components would be acceptable. and if more than 5% of the sample were larger or smaller than these size limits then the whole batch of components would be rejected.