This corresponds to a coefficient of variation in transmittance of up to 11 percent ( table I). When the test was repeated in 1974, the coefficients of variation among 135 laboratories still amounted to maximally 15 percent although 24 laboratories had been excluded by a special test because their photometers had more than 1 percent stray light at 240 nm. The results showed coefficients of variation in absorbance of up to 22 percent. A number of solutions was sent out and measured with spectrophotometers in 132 different laboratories. In 1973 the College of American Pathologists carried out a comparative test. In contrast, the transmittance of an object can be indicated to within 0.1 percent only, and even this allows merely statements as to random but not to systematic errors. There is no difficulty in stating refractive indices to within 5 decimals and tables of line spectra quote wavelengths to within 6 to 7 significant digits. The comparison of measured results of different optical parameters reveals considerable differences in accuracy.
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