An ad for ARCO graphite motor oil says (really): "Based on a 95% confidence level, our tests achieved between 1% and 8.7% mileage improvement" as compared with a conventional motor oil. What does the phrase "95% confidence level" mean here?
A) ARCO graphite beats 95% of conventional motor oils.
B) The interval from 1% to 8.7% covers 95% of the mileage improvements observed in the tests.
C) The tests included 95% of all oil brands on the market.
D) The estimate that mileage improves somewhere between 1% and 8.7% came from a method that would catch the true improvement in 95% of all similar tests.
E) A mistake has been made, because 95% + 8.7% is more than 100%.

Question

Quizplus · Accepted Answer

The phrase "95% confidence level" means that the estimate of 1% to 8.7% improvement came from a statistical method that is expected to catch the true improvement in 95% of all similar tests. It does not refer to the performance of other oils or the proportion of oils tested.

An Ad for ARCO Graphite Motor Oil Says (Really): "Based