Question

The table below shows a frequency distribution of the number of pickup trucks sold at 85 truck dealerships in Maine over an 18-month period.

Number of trucks sold/Number of dealerships:

70-90/2
90-110/11
110-130/39
130-150/17
150-170/9
170-190/7

Find the interval about the sample mean such that the probability is 0.90 that the true mean lies within the interval. (When P=90%, t=1.65)

I know that N=85, but I do not know how to find the standard deviation or mean in order to work the rest of the problem. Any help would be great.

Answer 1

SMALL-SAMPLE CONFIDENCE INTERVAL FOR A POPLATION MEAN

90% Confidence Interval = x-bar +/- 1.65 * s/SQRT(n)
n = NUMBER OF SAMPLES [3]

Cumulative Distribution Function

Student's t distribution with 3 DF

x P( X <= x )
1.65 0.901247