1
Stat 231 Exam II
October 31, 1997
Professor Vardeman
1.
ISU engineering statistics students conducted a study of the properties of liquid laundry
detergents.
In part of their study, they measured the amount of detergent that one needs to add to
45 ml of distilled water in order to reduce surface tension to the point that a particular small shirt
button floating on the surface of the water will sink.
Their data for two brands and some
summary statistics are below.
All Brand
ERA Brand
1.20, 1.05, 1.11
.85, .81, .69
1.120
.783
.075
.083
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(a)
What probability model assumptions must be made in order to do inference for the mean
amount of "All" detergent that must be added in order to reduce the surface tension?
(b)
Based on your assumptions from (a), give a 90% lower confidence bound for the mean
amount of All detergent required to reduce the surface tension.
(c)
Is there evidence of a difference in mean amounts of "All" and "ERA" required to reduce the
surface tension of distilled water, if one is willing to run a 5% chance of committing a type I
error?
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 Fall '08
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 Statistics, Normal Distribution, Standard Deviation, twosided confidence limits

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