3. Results
3.1. Descriptive statistics
For the Dutch study, data from 35 animals were lost
because the exact kidding date could not be retrieved or
because animals were missed during sampling. From the
Californian study, data from 22 does were missing, because
of loss to follow-up or missing samples. The total number
of animals in the datasets was therefore 495 for the
Dutch study and 416 for the Californian study. The number
of contaminated samples was 69 in the Dutch study and 16
in the Californian study. The apparent prevalence of IMI in
udder halves based on bacterial culture was 2.6% (73/2815)
in The Netherlands and 2.7% (64/2405) in California. For the
2 study sites, the respective apparent prevalence based on
SCC was 12.8% (367/2875) and 8.8% (214/2419) (Table 1).
The differences between apparent prevalence estimates
given in Table 1 and in the text are explained by the fact