Agriculture Reference
In-Depth Information
10.51 efg
10.00 fgh
9.84 gh
8.88 hi
8.71 i
he
duncan.do
command outputs the ANOVA table with the
means in descending order followed by letters signifying significant
differences. Means followed by the same letter are not significantly dif-
ferent at the 5% level. To see how Duncan's MRT differs from Scheffé's
Test load and run the
scheffe.do
command with the same data.
In addition, there are do files for calculating both Tukey's
w
pro-
cedure (
tukey.do
) and Student-Newman-Kuels' Test (
snk.do
)
available online.
*
Each uses a slightly different approach to determin-
ing significant differences.
Finally, there is a test for comparing a single treatment to all other
treatments called Dunnett's test. This has obvious applications in
agriculture, such as comparing a standard variety to new introduc-
tions or a common pesticide to new formulations. Several years ago,
I was interested in using this test. It had been proposed that this
test could be used as a Multiple Comparison with the Best (MCB).
This involved ranking the means in descending or ascending order
(depending if you were comparing to the largest value or the smallest
value) and choosing the top-ranked mean as the standard to compare
all other means. I was interested in comparing onion pungency data
in this fashion. I was not able to find Dunnett's
q'
values implemented
in Stata, so I called the company and they informed me that this was
not available. Several months later, I received an ado-file from Stata
that calculated Dunnett's
q'
. Dunnett's can now be accessed within
the
pwcompare
command, and the
dunnett
command is available
from the Web. Just type
findit
dunnett
. Open the dataset virus-
testinoc.dta, which as mentioned before is a dataset of watermelon
germplasm screened for resistance to zucchini yellow mosaic virus,
Egyptian strain (ZYMV-E). Values recorded for each entry are light
absorbance values from the colorimetric virus test. Enter the follow-
ing command and see the results:
dunnett
absorb trt,
control(
5
)
*
Files available online at http://www.crcpress.com/products/isbn/9781466585850.
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