A veterinarian was studying suspected causes of enteroliths

A veterinarian was studying suspected causes of enteroliths, concretelike balls in the colons of horses. She suspected that a diet of mostly alfalfa could be a cause. In a study conducted at a veterinary research facility she examined the records of 62 horses with enteroliths (cases) and compared them with the records of 75 horses free of enteroliths (controls). She found 42 case horses were fed mostly alfalfa compared with 17 control horses. She calculated a 95% confidence interval for (pcase – pcontrol), where pcase is the proportion of horses with enteroliths that are fed mostly alfalfa and pcontrol is the proportion of control horses mostly fed alfalfa and obtains (0.3007, 0.6008). Since this interval contains only positive values she concluded that the: case horses and control horses all get too much alfalfa. case horses and control horses get the same amount of alfalfa. proportion of case horses getting mostly alfalfa tis less than the control horses. proportion of case horses getting mostly alfalfa is larger than the proportion of control horses getting mostly alfalfa.

Solution

As this confidence interval is pcase - pcontrol, then pcase must be larger, as the whole interval is positive. Thus,

OPTION D: proportion of case horses getting mostly alfalfa is larger than the proportion of control horses getting mostly alfalfa.