Hicronj 1981, Vol.: 12, p.195. 0047-7206/8|/020|95-0|$02.00/0

©Pergamon Press Ltd. Printed in Great Britain.

MEASUR~ ERRORS IN ELECTRON MICROSCOPY

M. J. W. Webb National Vegetable Research Station, Wellesbourne, Warwick, U.K. CV35 9EF

This paper is concerned with the errors that may occur in taking measurements for calibrating electron microscopes and for sizing objects from negatives or printed enlarged micrographe. The inaccuracy of primary standards (some detected) or of errors occurring as a result of inconsistent operation of the electron microscopes are not considered.

A panel of six workers, three of which were familiar with making measurements from electron micrographe was established. Initially lines of differing lengths and orientation were measured using two different rulers. Later a pattern of 24 lines (6 identical) was measured using a modified caliper gauge. Finally a negative of a cross ruled diffraction grating and the resulting enlarged print were measured using the same caliper gauge. The results may be summarised as follows:

a. Each worker had a consistent standard error whatever pattern was measured.

b. Inexperienced workers bad larger standard errors than those familiar with measuring micrographs. Their measurements were both above and below the true values. The skilled workers had lower standard errors but consistently under measured.

c. Workers measuring with complicated rulers made more large errors. Measurements made just before breaks or late in the day tended to have more inaccuracies. All workers made outstandingly large errors on occasions and frequently failed to correct them. Few consistently measured the same length line accurately. Lack of concentration and fatigue appeared to contribute to the outstandingly large errors.

When the errors described interact with instrumental errors large inaccuracies occur. Standard errors should always be quoted in publications containing measurements of virus particles.

Ultimately, it is hoped to produce a standardised system for sizing virus particles using the electron microscope. This will incorporate not only a simplified statistical analysis to compensate for instrumental and manual measuring errors but will also compensate for both the inherent and artifactual variation of the virus particles themselves.

The help of Mrs D. G. Parfitt of the Statistics Section, NVRS, is gratefully acknowledged.

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