"The Nenana Ice Classic is an annual contest to guess the exact time in
the spring thaw when a tripod erected on the frozen Tanana Rver near Nenana,
Alaska, will fall through the ice."  (Moore, Notz, Fligner p. 191).  

The data in tanana.dat are taken from the Nenana Ice Classic from the years
1917 - 2010.  For each year, the date on which the tripod fell each year is
recorded as the number of days before or after Apr 19 (i.e. Apr 20 is 1,
Apr 21, is 2, etc.).  The variables are:

year -- year
day  -- date of fall as number of days before or after Apr 19.
group -- years grouped as in question 7.44 (A = 1917-39, B = 1940-62,
         C = 1963-85, D = 1986-2010)