In an attempt to save some cut and paste time, the data used in this midterm is in a spreadsheet. If viewing this page online, right-click with the right mouse button on the link below and choose "Save target as... to save this file to your computer. The file can then be opened with Excel.
http://www.comfsm.fm/~dleeling/statistics/s43/mdata.xls • html format
Under-inflation is not only the tire's enemy for durability but is a significant contributor to increased rolling resistance and loss of fuel economy. Under-inflation by as little as 20% can reduce tire life up to 50% and increase fuel consumption by 3%. Fifteen tires with a recommended air pressure of 35 pounds per square inch (PSI) were randomly measured for their air pressure. The measurements are given in the table.
|Bins (x)||Frequency (f)||Rel. Freq. p(x)|
Treadwear. In 1973 tire treads lasted for roughly 24,000 miles. Today most passenger car tires average about 41,000 miles before their treads wear out. The United States Department of Transportation rates tires using a grading system. A control tire is assigned a grade of 100. A tire with a grade of 200 can be expected to last twice as long as the control tire, while a tire with a grade of 80 is normally less durable.
The USDOT has data on 2397 tires available on their web site. Use the table below to work out the mean grade for the 2397 tires.
|Grade bins||Freq||RF or P(x)||x*p(x)|
Goodyear P205/70R14 light truck tires are to have their inflation pressure adjusted according to the load. Goodyear provides the following tire load limits (lbs) at the shown cold inflation pressures.
|Statistic or Parameter||Symbol||Equations||Excel|
|Sample standard deviation||sx||=STDEV(data)|
|Sample Coefficient of Variation||CV||sx/x||=STDEV(data)/AVERAGE(data)|
|Slope||b||=SLOPE(y data, x data)|
|Intercept||a||=INTERCEPT(y data, x data)|
|Correlation||r||=CORREL(y data, x data)|
|Coefficient of Determination||rČ||=(CORREL(y data, x data))^2|