Ma. Faust et Ll. Timms, ESTIMATES OF VARIABILITY FOR SOMATIC-CELL COUNT MEASUREMENTS IN THE IOWA DAIRY-INDUSTRY, Journal of dairy science, 78(3), 1995, pp. 546-551
The SCC collected from 15 instruments in 12 laboratories were used to
quantify accuracy, repeatability, and reproducibility in the Iowa dair
y industry. For each of three trials, milk was sampled at the morning
milking from 30 different Holsteins in the Iowa State University herd.
Identified samples and unidentified duplicates were provided for each
participating instrument. Mean SCC was 418,000 cells/ml, and mean SCC
for duplicates ranged from 9000 to 3,966,000. Accuracy for a set of 3
0 duplicates was lowest for trial 1 (CV = 16.4%) and highest for trial
2 (CV = 7.6%). Intraclass correlations estimated repeatability and we
re .99 for all but one instrument. Coefficients of variation for repea
tability (weighted mean = 11.4%) were similar to estimates for accurac
y (weighted mean = 11.0%), but reproducibility was considerably lower
(30.0%). Samples were classified by SCC as very low, <125,000; low, 12
5,000 to 249,000; medium, 250,000 to 500,000; and high, >500,000. Repe
atability for high samples was higher than repeatability for very low
SCC samples; coefficient of variation for high SCC samples was 6.8% bu
t was >25.0% for samples with <500,000 cells. Repeatability was within
standards set by the industry, but current procedures for quality con
trol may not adequately address reproducibility.