Two brands of water filters were evaluated in terms of their mean abilities to reduce impurities (measured in parts per million, ppm). The sample summary statistics are:

Sample size Sample mean Sample variance

Filter Brand 1 21 8 4.5

Filter Brand 2 21 6.5 2

Let 1 denote the population mean level of impurity reduction for brand 1 and let 2 denote the population mean level of impurity reduction for brand 2.

a. Calculate a point estimate of 1- 2.

b. Calculate a 95% confidence interval for 1- 2.

c. Which brand appears to be better at removing impurities in water, on average? Explain.

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