9.18 The ages and annual expenditure on traditional suits by a random sample of eight males belonging to the AB socioeconomic class were:

Age 27 29 33 34 39 42 49 56 Expenditure (£) 280 400 650 800 1200 1100 1250 1800

(a) Regress expenditure against age and test the hypothesis of a zero population slope at the 10 per cent level of significance.

(b) Produce a 90 per cent confidence interval for 35-yearold AB class males’ mean annual expenditure on suits.

(c) Produce a 90 per cent confidence interval for 75-yearold AB class males’ mean annual expenditure on suits.

(d) Compare your answers to

(b) and (c). Which is the more valid, and why?