Question

Fizzy Waters has been involved in multiple lawsuits regarding the alkaline substance concentration in their product and getting in trouble with clients about the numerous health claims they have been making. The last thing they want to deal with is not delivering on the advertised amount of content. They want to ensure that the filling process of their 160 oz (1.25 gal) bottles of alkaline water is not highly variable. The quality control engineer at Fizzy Waters suggests repairing the filling machine if the variance of the process is more than 25 oz. 8 random samples were collected and their content in ounces (oz) is given in the table below.

158.2 162.8 161.5 161.2 166.5 160.1
158.4 175.6 159.9 168.8 161.9 163.7
Perform a hypothesis test with 90% reliability to determine whether the filling machine needs to be repaired. Based on the test, explain why the filling machine needs or does not need to be repaired.

Answer 1

Explanation:

For the sample, n = 12

Mean, x = (158.2 + 162.8 + 161.5 + 161.2 + 166.5 + 160.1 + 158.4 + 175.6 + 159.9 + 168.8 + 161.9 + 163.7)/12 = 163.22

Variance, s² = (summation(x - mean)²/n

Summation(x - mean)² = (158.2 - 163.22)^2 + (162.8 - 163.22)^2 + (161.5 - 163.22)^2 + (161.2 - 163.22)^2+ (166.5 - 163.22)^2 + (160.1 - 163.22)^2 + (158.4 - 163.22)^2 + (175.6 - 163.22)^2 + (159.9 - 163.22)^2 + (168.8 - 163.22)^2 + (161.9 - 163.22)^2 + (163.7 - 163.22)^2 = 273.5368

Variance, s² = 273.5368/12 = 22.79

This is a test for a single variance. We would set up the test hypothesis.

For the null hypothesis,

H0: σ² ≥ 25

For the alternative hypothesis,

H1: σ² < 25

The formula for determining the test statistic,x² is

x² = (n - 1)s²/σ²

Where n - 1 is the degree of freedom, df.

df = 12 - 1 = 11

x² = (11 × 22.79)/25 = 10.0276

For a test of 90% reliability, Confidence level = 0.9

Cl = 1 - alpha(level of significance)

alpha = 1 - 0.9 = 0.1

The critical value from the chi-square distribution table is 17.28. Since 17.28 > 10.0276, we would reject the null hypothesis. Therefore, the filling machine does not need repair because the variance of the process is not more than 25 oz.