Every year, Nenana, Alaska, hosts a contest in which participants try to guess the exact minute that a wooden tripod placed on the frozen Tanana River will fall through the breaking ice.The contest started in 1917 as a diversion for railroad engineers with a jackpot of $800 for the closest guess.It has grown into an event in which hundreds of thousands of entrants enter their guesses on the Internet and vie for more than $300,000.

Because so much money and interest depends on the time of breakup, it has been recorded to the nearest minute with great accuracy ever since 1917.And because a standard measure of breakup has been used throughout this time, the data are consistent.An article in Science ("Climate Change in Nontraditional Data Sets."Science 294 [26 October 2001]: 811) used the data to investigate global warming - whether greenhouse gasses and other human actions have been making the planet warmer.Others might just want to make a good prediction of next year's breakup time.

Can we estimate the rate of change (if any) and use it to make better predictions? Here are some of the data: Breakup date Breakup date Year (days after Year (days after Jan 1) Jan 1) 1917 119.4792 1930 127.7938 1918 130.3979 1931 129.3910 1919 122.6063 1932 121.4271 1920 131.4479 1933 127.8125 1921 130.2792 1934 119.5882 1922 131.5556 1935 134.5639 1923 128.0833 1936 120.5403 1924 131.6319 1937 131.8361 1925 126.7722 1938 125.8431 1926 115.6688 1939 118.5597 1927 131.2375 1940 110.6437 1928 126.6840 1941 122.0764 1929 124.6535 ... Is there evidence at the 5% significance level that as the years increase, the number of breakup days decreases