Question

Dairy cows at large commercial farms often receive injections of bST (Bovine Somatotropin), a hormone used to spur milk production. Bauman et al. (Journal of Dairy Science, 1989) reported that 12 cows given bST produced an average of 28.0 kg/d of milk. Assume that the standart deviation of milk production is 2.25 kg/d. (a) Find a 99% confidence interval for the true mean milk production. Round your answers to two decimal places (e.g. 98.76).

Answer 1

The 99% confidence interval for the true mean milk production is between 26.33 kg/d and 29.67 kg/d.

Explanation:

We have that to find our
`\alpha` level, that is the subtraction of 1 by the confidence interval divided by 2. So:

`\alpha = (1 - 0.99)/(2) = 0.005`

Now, we have to find z in the Z-table as such z has a p-value of
`1 - \alpha`.

That is z with a pvalue of
`1 - 0.005 = 0.995`, so Z = 2.575.

Now, find the margin of error M as such

`M = z(\sigma)/(√(n))`

In which
`\sigma` is the standard deviation of the population and n is the size of the sample.

`M = 2.575(2.25)/(√(12)) = 1.67`

The lower end of the interval is the sample mean subtracted by M. So it is 28 - 1.67 = 26.33 kg/d.

The upper end of the interval is the sample mean added to M. So it is 28 + 1.67 = 29.67 kg/d.

The 99% confidence interval for the true mean milk production is between 26.33 kg/d and 29.67 kg/d.