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VIII. STATISTICAL ASPECTS OF COLOR SENSITOMETRY
The nature of sensitometric measurements and the materials on which they are made give rise to random variations in the results. In the presence of these random variations precise determinations are not possible on the basis of a single experiment. Reliable conclusions can be drawn, however, from multiple experiments interpreted by statistical methods. Experienced operators have long realized the distinction between significant and insignificant differences in results, but the quantitative study of these variations and of their bearing on control problems is a relatively recent development. Within the limits of the present report, these methods cannot be fully described nor even adequately outlined; the purpose of this final section is simply to call attention to the existence of this new and important phase of color sensitometry.
Statistical Variation of Results
When several duplicate exposures are made on a small sample of photographic material, and the group is processed together under carefully controlled conditions, and then measured on a correctly adjusted densitometer, the individual images will not have identical densities. If the sensitometric test procedure is well controlled, these density variations will represent random variations for which no specific causes can be assigned.
Variations greater than these random variations, for which assignable causes can be found, will often be observed when (a) different coatings of photographic material are tested, (b) the material is exposed on different sensitometers or on the same sensitometer at different times, (c) the strips are processed at different times, and (d) the processed strips are read on different densitometers or on the same densitometer at different times. An important problem of color sensitometry is the evaluation of such systematic differences from data which contain the random variations.
Individual Sources of Variation
Among these different components of systematic variability the only one that can be isolated for independent study is the densitometric variation. Its magnitude can be determined by measuring the same processed sensitometric strip on several densitometers, or on the same densitometer at various times. An objective of good densitometer design and of sound maintenance and control procedures is the
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