* Mendota Ice Exercises
*----------------------

* Setup:  read the data into Stata with these commands
infile str7 winter str6 close str6 open days using "http://www.ssc.wisc.edu/~hemken/Stataworkshops/Mendota2011.raw"
generate year=real(substr(winter, 1, 4))

* Problem 1
*----------
* What what was the longest ice fishing season on record?  
*   How many days long was it, and what was the year? 
*   What was the shortest season?

* Problem 2
*----------
* How common has it been to have 120 or more days of ice cover?
*
*   (Hint: create an indicator.)

* Problem 3
*----------
* Would a person who grew up in the period 1900-1920 (like my
*   grandparents) be justified in thinking that "today's kids
*   have it easy"?  Compare the mean duration of ice cover for
*   1900-1920 versus 1990-2010.  
*
*   (Hint:  You could drop unneeded observations with a "drop if" 
*   command, or filter them with an indicator variable and
*   an "if" clause.)

* Problem 4
*----------
* When should we expect the Lake to open this spring?
*
* (Hint:  convert the variable "open" to a day-of-the-year
*    by using the mdy() function or the date() function
*    and the %td format.  Just getting the components for 
*    mdy() or date() will be the hardest part of this
*    exercise.  Use a constant year.  Take the mean for a
*    simple prediction, and construct an optional confidence 
*    interval.
*    If you know regression, predict a result for "2010".