Student's ttest
Use Student's ttest when you have one attribute variable and one measurement variable,
and you want to compare the mean values of the measurement variable. The attribute
variable must have only two values, such as "male" and "female" or "treated" and
"untreated."
Example:
In fall 2004, students in the 2 p.m. section of Biological Statistics had an
average height of 66.6 inches, while the average height in the 5 p.m. section was 64.6
inches. Are the average heights of the two sections significantly different?
Oneway anova
Analysis of variance (anova) is the most commonly used technique for comparing the
means of groups of measurement data. In a oneway anova there is one measurement
variable and one attribute variable. Multiple observations of the measurement variable are
made for each value of the attribute variable.
all anovas, assumes that the observations within each cell are normally distributed
and have equal variances
.
Example
:
you could measure the amount of transcript of a particular gene for multiple
samples taken from arm muscle, heart muscle, brain, liver, and lung. The transcript
amount would be the measurement variable, and the tissue type (arm muscle, brain, etc.)
would be the attribute variable.
KruskalWallis test and MannWhitney U test
The KruskalWallis test is used when there is one attribute variable and one measurement
variable, and the measurement variable does not meet the
normality
assumption of
ANOVA. It is the nonparametric analogue of a
oneway ANOVA.
A oneway
ANOVA may yield inaccurate estimates of the Pvalue when the data are very far
from normally distributed. The KruskalWallis test does not make assumptions
about normality. It is performed on
ranked data,
so the measurement observations
are converted to their ranks in the overall data set: the smallest value gets a rank of
1, the next smallest gets a rank of 2, and so on.
The MannWhitney Utest or the Wilcoxon test or the Wilcoxon ranksum test is limited
to attribute variables with only two values; it is the nonparametric analogue to
Student's ttest.
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View Full DocumentTwoway anova
Use a twoway ANOVA when you have one measurement variable and two attribute
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 Spring '08
 buendia
 Statistical tests, Nonparametric statistics, Attribute Variables

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