A recent study of food portion sizes reported that over a 17-year period, the average size of a soft drink consumed by Americans aged 2 years and older increased from 13.1 ounces (oz) to 19.9 oz. The authors state that the difference is statistically significant with P < 0.01.† Explain what additional information you would need to compute a confidence interval for the increase, and outline the procedure that you would use for the computations.