234
difference between the two groups was not significant when assessed by the logrank test for equality of survivor functions (P = 0.097) but significant according to
the Wald chi-square statistic (P = 0.032).
The mean number of days from calving to pregnancy was 220 (SD = 202) for
first parity cows and 170 (SD = 156) for multiparous cows. Figure 2 displays the
Kaplan–Meier plot of first parity versus multiparous cows by days after calving.
no bull
bull
0.00
0.25
0.50
0.75
1.00
0
200
400
600
time (days after calving)
probability of not conceiving
Fig. 1 Kaplan–Meier plot of the proportion of pregnant cows distributed by days after calving
between cows from a herd with a breeding bull and cows in herds without a bull
first parity cows
multiparous cows
0.00
0.25
0.50
0.75
1.00
0
200
400
600
time (days after calving)
probability of not conceiving
Fig. 2 Kaplan–Meier plot of proportion of pregnant cows distributed by days after calving
between first parity and multiparous cows
M. A. Bhatti et al.