EXAMPLE 12.23

Confidence interval for the second contrast. To find the 95% confidence interval for ψ₂, we combine the estimate with its margin of error:

= 0.91 ± 0.47

The interval is (0.44, 1.38). Unfortunately, this interval is difficult to interpret because the units are . We can obtain an approximate 95% interval on the orginial units scale by back-transforming (squaring the interval end points). This results in an approximate 95% confidence interval of the difference to be between 0.19 minutes and 1.90 minutes.