**Question: **(10 marks). It is suspected that brushing teeth straight after eating a meal has a detrimental effect on enamel loss. To explore this theory, 15 pairs of twins performed two tests. One of the twins (twin 1) did tooth brushing 30 min after each meal, while the other twin (twin 2) brushed teeth straight after the meal. Measurement taken is the erosion of enamel (μm) after 12 months (compared with the baseline). The data are stored in the file **data1_Q4.sav**.

a. Should we perform a paired sample t-test, or a two-sample independent t-test on the data set? (2 marks).

b. Using the test of your choice, state the null hypothesis and the alternative hypothesis. (2 marks)

c. Check the normality of the data (Hint: should we check the normality of enamel erosion of each twin, or should we check the normality of the difference between the enamel erosion of the twin pairs?) (2 marks)

d. Assume the normality condition is satisfied, what is the statistical result of the test of your choice? (2 marks)

e. What is your final conclusion? (How you interpret your finding to general public?) (2 marks)