The owner of a record store has advertised his business in a popular search engine. Before ad- vertising, he collected a sample of the sales during eight randomly selected days. The observed sales (in SEK) were: 8100 6000 10600 10000 5800 8900 7000 8700 After advertising, he collected a sample of the sales during twelve randomly selected days. The observed sales (in SEK) were: 13500 11500 12800 12400 7800 11300 10000 10000 3500 11000 9400 8900 The owner knows, by experience, that the sales can be adequately described by a normal distribution. In addition, he considers it safe to assume that the variance before and after advertising is the same. (a) Find a 95% confidence interval for the expected change in sales (after minus before). (5p.) (b) Using a significance level of 5%, test the null hypothesis that there was no change in sales against the two-sided alternative. i. State the hypothesis of interest; ii. Compute the test statistic and the critical value; iii. What is the conclusion regarding the hypothesis? (5p.) (c) Consider the null hypothesis that, after advertising, the sales increased by 0 SEK against the two-sided alternative. Using a significance level of 5%, for what values of po would the null hypothesis not been rejected. (5p.) (d) Compare your answers in (a) and (c). What do you conclude? (5p.)