| Economics has long been known as an unpleasant science. But is any economist so dull as to criticize Christmas？ At first glance, the holiday season in western economies seems a treat for those who are concerned with such things as GDP growth. After all, everyone is spending; in America, sellers make 25 % of their yearly sales and 60 % of their profits between Thanksgiving and Christmas. Even so, economists find something to worry about in the nature of the purchases being made. Much of the holiday spending is on gifts for others. At the simplest level, giving gifts involves the giver's thinking of something that the receiver would like - he tries to guess her preferences, as economists say - and then buying the gift and delivering it. Yet this guessing of preferences is often done badly. Every year, ties go unworn and books unread. And even if a gift is enjoyed, it may not be what the receiver would have bought if they had spent the money themselves. Interested in this mismatch between wants and gifts, in 1993 Joel Waldfogel, then an economist at Yale University, attempted to estimate the disparity(差距) in dollar terms. He asked students two questions at the end of a holiday season: first, estimate the total amount paid (by the givers) for all the holiday gifts you received; second, apart from the emotional value of the items, if you did not have them, how much would you be willing to pay to get them？ His results were unpleasant: on average, a gift was valued by the receiver well below the price paid by the giver. The most conservative(保守的) estimate put the average receiver's valuation at 90% of the buying price. The missing 10% is what economists call a deadweight loss(无谓损失)because it's a waste of resources that could be avoided without making anyone poorer. In other words, if the giver gave the cash value of the purchase instead of the gift itself, the receiver could then buy what he/she really wants and be better off for no extra cost. It suggests that in America, where givers spend $40 billion on Christmas gifts, $ 4 billion is being lost annually in the process of gift giving. Add in birthdays, weddings and non Christian occasions, and the figure would balloon. So should economists call for an end to gift giving, or at least press for money to become the gift of choice？ | 
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, the pooled sample variance that should be used in the pooled-variance t test.
n1 = 16, n2 = 25.
