Question

In Tucson, Arizona, the air pollution index averages 62.5 during the year with a standard deviation of 18. assuming normality, the index falls within what interval 95% of the time?

A) (8.5, 116.5)
B) (44.5, 80.5)
C) (45.4, 79.6)
D) (26.5, 98.5)
E) not enough info given

Answer 1

The interval of 95% confidence, for normally distributed variables, is average +/- 2 * standard deviation:

(-2*standard deviation + average , average + 2*standard deviation)

(-2*18 + 62.5 , 2*18 + 62.5) = (-36 + 62.5 , 36 + 62.5) = (26.5 , 98.5)

Answer: option D)