A series of experiments is described, outlining a study of 5 factors likely to influence the efficiency and reproducibility of a standardized method used for the electronic counting of somatic cells in milk. Samples containing cells covering 5 ranges from 250000 cells/ml to 1500000 cells/ml were used and the results analysed statistically.

The residual error was given special consideration, and was found not to contribute to any significant degree to the accuracy of the method. Sample volumes of 0·5 ml and 0·1 ml used for counting were compared, and there was a decrease in accuracy using the smaller volume. The influence of individual technician skill was studied with 4 workers having differing degrees of experience, and no significant addition to the overall error was observed. The effect of agitating bulk tank milk for between 0 and 2 min before sampling was examined. Variations were considerable for agitation times of less than 1 min, but the recommended mixing time of 2 min was found to be adequate with milk quantities between 450 l and 2250 l (100 and 500 gal), and at the 5 cell-count levels chosen.

The distributions of the cell counts at the 5 levels were found to be normal with the marginal exception of samples tested at 250000 cells/ml.

All aspects of the technique, sample preparation, use and calibration of the Coulter Counter, etc, should be closely followed to ensure that no inaccuracies will result from inadequate maintenance of the machine or other equipment employed in the test.