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STAT 427
Solution to Midterm I
Winter, 2004
Prof. Goel
MWF 10:30  11.20
NOTES: This is a closed book examination.
You can use your own calculator and a singlesided 8.5” x 11” crib sheet.
There are four problems worth a total of 50 points.
You must explain each step in your answer to get partial credits. Good Luck!
Problem 1.
[10 points] Environmental impact studies on the quality of water often measure the
level of chemical toxicants by the LC50, the concentration lethal to 50% of the organisms of a
particular species under a given set of conditions and specified time period. Two common
toxicants are methyl parathion (MP) and Baytex. LC50 measurements on water samples for each
of these toxicants are as follows:
MP
3.1, 4.7, 5.2, 2.8, 5.2, 5.3, 5.7, 6.6, 5.7, 7.1, 9.0, 8.9
Baytex
0.9, 1.2, 1.3, 1.4, 1.3, 1.6, 1.5, 1.6, 1.7, 1.9, 3.4, 2.4
(i) [6 points] Calculate the medians, the lower and upper quartiles of the above data sets.
First one must order the data from the smallest to the largest as follows:
MP
2.8, 3.1, 4.7, 5.2, 5.2, 5.3, 5.7, 5.7, 6.6, 7.1, 8.9, 9.0
Baytex
0.9, 1.2, 1.3, 1.3, 1.4, 1.5, 1.6, 1.6, 1.7, 1.9, 2.4, 3.4
Since n = 12, the median is the average of 6
th
and 7
th
items, the lower quartile is the
average of 3
rd
and 4
th
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 Spring '09
 Statistics, Quartile, Th, Quantile

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