Study dairy farm and sample size calculation
Blood samples were collected by venipuncture from 350 randomly selected
dairy cattle that belonged to a herd of 6000 Holstein-Friesian cattle.
The study farm is located in the Al Ain region, Emirate of Abu Dhabi,
UAE and up until the time of the research, the dairy farm had been
experiencing abortion problems. The sample size was determined using the
formula, \(n=z_{\alpha}^{2}pq/L^{2}\) (Thrusfield, 2007,
3rd Edition, pp. 283-240): where, \(n=\)sample size,\(z_{\alpha}=\) normal deviate (1.96) at 5% level of significance,\(p\ =\) estimated prevalence, \(q=1-p\) and \(L=\) precision of
estimate usually at 5%. For the sample size calculation, a priori
bovine coxiellosis prevalence of 22.3% reported in Iran (Azizzadehet al. 2012).
The sample size was therefore derived as follows, \(n\):
\(n=\left(1.96\right)^{2}\times 0.223\times\frac{1-0.223}{\left(0.05\right)^{2}}=\approx 311\)
To adjust for potential non-compliance and design effect, the calculated
sample size was increased to 350. Following collection, the blood
samples were allowed to clot at room temperature (rT), the sera
separated by centrifugation at 4000 rpm, 5 min and then kept at\(-20C^{\circ}\) until testing.