Answer to Question #191938 in Statistics and Probability for Sizwe

Question

Suppose that 3000 randomly breath-tested, 4000 were tested and 98 were above limit

Answer 1

After a month, the proportion was: $\frac{98}{4000}=0.0245$

1. Before we draw any conclusions from these results, we must ensure that the samples are fairly similar, as if we have too many non-drinkers, the rate will inevitably fall.
2. Other than a genuine shift in driver behavior, there are a number of variables that could contribute to a decrease in the proportion of drivers who exceed the speed limit. These are: 

(I).Perhaps the samples were skewed by the presence of a certain sort of person in large numbers.

(II). Breath test loss, resulting in a false negative.

(III). Other considerations, such as enforcement of anti-drink-and-drive laws.