Snowfall Project Update – December 20, 2000

Name:________________________

Below are the means and population standard deviations for the snowfall data we used in our first project. I have not included 1996 data since the data was not complete for the whole year.

The “population” standard deviations are what we have learned how to calculate, for this case the “sample” standard deviations would be slightly more accurate but not by much. The sample standard deviation uses N-1 instead of N in the denominator of the squared differences term. Since this is a little smaller than N you would expect the sample standard deviation to be a little larger (smaller divisor means larger quotient). This is the case as the sample standard deviations are between .001 and .1 inches higher than the corresponding population standard deviations, a difference we can ignore.

Questions:

1. Would you expect the monthly snowfall data to be “normally” distributed? Why or why not?

2. On a separate sheet, make a histogram (frequency chart) of the December snowfall data. Does this support your conclusion for question 1? Why or why not?

3. On a separate sheet, make a histogram (frequency chart) of the total annual snowfall data. Does this support your conclusion for question 1? Why or why not?

4. As of December 20, 2000 we have received 19.8 inches of snow ⁽¹⁾ with more predicted! Using what you now know about statistics and measures of variance, how unusual is our December snowfall this year given that our average December snowfall from 1961 to 1995 was only 8.4 inches? Your answer should include numbers to support your conclusions.

⁽¹⁾ O’Hare December Snowfall. Reference: http://iwin.nws.noaa.gov/iwin/il/climate.html