Question

The manufacturer of official NFL footballs uses a machine to inflate its new balls to a pressure of 13.5 pounds. When the machine is properly calibrated , the mean inflation pressure is 13.5 pounds, but uncontrollable factors cause the pressures of individual footballs to vary randomly from 13.3 to 13.7 pounds. For quality control purposes, the manufacturer wishes to estimate the mean inflation pressure to within 0.025 pound of its true value with a 99% confidence interval. What sample size should be used

Answer 1

106

Explanation:

Given that:

Mean (m) = 13.5 pounds

Error margin = 0.025

α = 99%

Variation interval = range = 13.3 to 13.7

Standard deviation (σ) estimate from range :

Range / 4

(13.7 - 13.3) /4

= 0.4 / 4

= 0.1

Zcritical at 99% = 2.576

Using the formula :

Sample size (n) = [ (Zcritical * σ) / M. E]^2

n = [(2.576 * 0.1) / 0.025]^2

n = (0.2576 / 0.025)

n = 10.304^2

n = 106.172

n = 107 samples