Question

Applications of Confidence Intervals

I surveyed 20 co workers to see what cell phone provider they use.

Phone Provider	Population	Percentage
At&T	3	15%
T-Mobile	3	15%
Verizon	6	30%
Metro PCS	6	30%
Sprint	2	10%

The confidence level I will use is 85%. What is the critical interval for Verizon customers?

The confidence level I will use is 80%. What is the critical interval for Verizon customers?

Please show me what steps you took...this topic is really confusing for me!

Answer 1

I assume you want confidence interval for proportion of population who use Verizon

confidence interval for proportion is

(p^ - z * sqrt(pq/n) , p^ + z * sqrt(pq/n))

z is critical value

Now

you can find critical value using normal distribution table or using software like R,SPSS,Minitab

Using R

for 85% confidence level

z= =NORMSINV(1-(1-0.85)/2)

=1.4395

b)

for 80%

z = =NORMSINV(1-(1-0.8)/2)

= 1.2816

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