“Sample size calculations were based on a published prevalence of IMI at calving
of approximately 20% when both DCT and ITS were used at drying off (Godden
et al., 2003; Sanford et al., 2006a). With a significance level of 95% (2-sided) and
a power of 80%, it was determined that approximately 2,280 quarters (570 cows
with 285 being in each group) would be required to detect a minimum change in
risk of IMI at calving from 20 to 25% (margin of noninferiority of 5%). This
calculation assumes that quarters within a cow are independent, although
research has shown interdependence of quarters with respect to IMI (Barkema et
al., 1997; Berry and Meaney, 2006). To account for clustering of quarters within
cows, an adjustment to the sample size was made using a published intraclass
correlation coefficient (ρ) of 0.20 (Barkema et al., 1997; Berry and Meaney,
2006), resulting in a sample size of 3,632 quarters (908 cows, with 454 per
group).”