A fast-food chain decided to carry out an experiment to assess the influence of advertising expenditure on sales. Different relative changes in advertising expenditure, compared to the previous year, were made in eight regions of the country, and resulting changes in sales levels were observed. The accompanying table shows the results.

a. Estimate by least squares the linear regression of increase in sales on increase in advertising expenditure.
b. Find a 90% confidence interval for the slope of the population regression line.

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Increase in advertising expenditure (%) Increase in sales (%) 0 41410 986 1 24 7.2 10.3 9.1 10.2 4.1 7.6 3.5