On 10/14/2011 1:20 PM, Weidong Gu wrote:
On Fri, Oct 14, 2011 at 11:38 AM, Michael Friendly<frien...@yorku.ca> wrote:
Hi all
Consider the classic data below from Darwin on the heights of 15 pairs of
zea mays (corn) plants
either cross-fertilized or self-fertilized, where the goal is to see if it
makes a difference.
head(ZeaMays)
pair pot cross self diff
1 1 1 23.500 17.375 6.125
2 2 1 12.000 20.375 -8.375
3 3 1 21.000 20.000 1.000
4 4 2 22.000 20.000 2.000
5 5 2 19.125 18.375 0.750
6 6 2 21.500 18.625 2.875
...
I'd like to illustrate two types of non-parametric tests of whether the
mean(diff) = 0.
(a) Permutation test, where the values of, say self are permuted and
diff=cross - self
is calculated for each permutation. There are 15! permutations, but a
reasonably
large number of random permutations would suffice.
(b) Test based on assigning each abs(diff) a + or - sign, and calculating
the mean(diff).
There are 2^15 such possible values, but again, a reasonably large number of
random
samples would do.
What do you mean by 'assigning each abs(diff) a + or - sign, and
calculating the mean(diff)'?
abs(diff) should be all positive, right?
I mean to calculate
mean (sign * abs(diff))
where sign is a vector of length 15 composed of some combination of (-1,
+1).
This is actually what Fisher did. It corresponds to assigning one
member of each pair to cross / self
--
Michael Friendly Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street Web: http://www.datavis.ca
Toronto, ONT M3J 1P3 CANADA
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