Seasonal spending. The success of the Christmas spending season (in the United States, roughly from Thanksgiving to Christmas) drives much of retail sales for the year. Financial analysts for credit card companies know that January credit card charges will generally be much lower than the month before and so must plan for the decrease in revenue. A simple random sample of 99 cardholders spending for each month from December 2004 to February 2005 can be found in the data set Ch25_Seasonal_Spending. Construct a 95% confidence interval for the mean change in charges between December and January.