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1950 REFERENCES 723
licate sensitometric strips at 2j/£-min intervals for about one hour in a single processing machine. The present illustration is concerned only with the blue integral densities of one specific step on each strip. The rational subgroups, in this experiment, are the groups consisting of the two strips processed at the same time. The density difference between two such strips (the "range" of duplicate strips) is taken as a measure of the variability within each subgroup. The lower graph in Fig. 14 shows this quantity for each of the 21 subgroups. The dotted line on this lower graph indicates a statistical control limit based on the average range. Within the duration of this experiment, all ranges of duplicate strips are within the control limit ; this is interpreted to mean that differences observed between duplicate strips processed together are due to nonassignable chance causes.
This minimum random variation also provides a basis for calculating the control limits shown by dotted lines in the upper graph of Fig. 14. Here, average densities of each set of two strips are plotted. As long as these average densities are within the control limits, the variations observed in the result may be considered as random effects from unassignable causes. The upper graph shows, however, that in the test under discussion the variations extend beyond the control limits. This is interpreted to mean that the downward trend shown during the hour may have had non-random origin. Such a situation would justify a careful search for specific causes of trouble.
REFERENCES
1. C. E. K Mees, The Theory of the Photographic Process, p. 616, Macmillan,
New York; 1945.
2. L. A. Jones and C. A. Morrison, "Sensitometry of photographic papers,"
/. Frank. Inst., vol. 228, pp. 445^70; October, 1939.
3. W. Bornemann and C. Tuttle, "An intensity-scale sensitometer that
works at intensity-time levels used in practical photography," J. Opt. Soc. Amer., vol. 32, pp. 224-229; April, 1942.
4. ASA Z38.2.5 (1946), "American standard for diffuse transmission density,"
pp. 18-22; American Standards Association, 70 E. 45th St., New York 17.
5. R. N. Wolfe and R. S. Barrows, "Adjacency effects in photography," /.
Phot. Soc. Amer., vol. 13, pp. 554-556; September, 1947.
6. M. H. Sweet, "A simple intensity scale sensitometer which conforms with
American Standard requirements," /. Opt. Soc. Amer., vol. 35, pp. 379381; June, 1945.
7. R. Davis and K. S. Gibson, "Filters for the reproduction of sunlight and
daylight and the determination of color temperature," Misc. Publ., Bur. Stand., No. 114, 165pp.; 1931.
8. D. R. White, "Two special sensitometers," Jour. SMPE, vol. 18, pp. 279
291; March, 1932.