Question

Traffic police monitor the speed of vehicles as they travel over a new bridge. The average speed for a sample of 4141 vehicles was 81.3581.35 km/h, with the sample standard deviation being 4.524.52km/h. We will assume that the speeds are Normally distributed, and the police are interested in the mean speed.

Part a) Since the variance of the underlying Normal distribution is not known, inference here would involve the t distribution. How many degrees of freedom would the relevant t distribution have?

Part b) Create a 95 % confidence interval for the mean speed of vehicles crossing the bridge. Give the upper and lower bounds to your interval, each to 2 decimal places. (  ,)

Part c) The police hypothesized that the mean speed of vehicles over the bridge would be the speed limit, 80 km/h. Taking a significance level of 5 %, what should infer about this hypothesis?

A. We should not reject the hypothesis since the sample mean is in the interval found in (b).
B. We should reject the hypothesis since 80 km/h is in the interval found in (b).
C. We should reject the hypothesis since the sample mean was not 80 km/h.
D. We should reject the hypothesis since 80 km/h is not in the interval found in (b).
E. We should not reject the hypothesis since 80 km/h is in the interval found in (b).

Part d) Decreasing the significance level of the hypothesis test above would (select all that apply)

A. decrease the Type I error probability.
B. either increase or decrease the Type I error probability.
C. not change the Type II error probability.
D. increase the Type I error probability.
E. not change the Type I error probability.

Answer 1

a)

degrees of freedom=n-1 =40

b)

sample mean	x̄=		81.3500
sample size	n=		41.00
sample std deviation	s=		4.520
standard error of mean =	sx=s/√n=		0.7059

for 95% CI; and 40 degree of freedom, value of t=					2.021
margin of error E=t*std error                            =					1.4266
lower confidence bound=sample mean-margin of error =					79.923
Upper confidence bound=sample mean+margin of error=					82.777
from above 95% confidence interval for population mean =(79.92,82.78)

c)

E. We should not reject the hypothesis since 80 km/h is in the interval found in (b).

d)

A. decrease the Type I error probability.