In this real example (below), all four of the replicates in one
treatment combination had zero failures, and this produced a very high
standard error in the summary.lm.
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Just adding one failure to one of the replicates produced a well-behaved
standard error.
=20
I don't know if this is a bug, but it is certainly hard for users to
understand.
=20
I would value your comments=20
=20
Thanks
=20
Best wishes,
=20
Mick
=20
Prof M.J. Crawley
=20
Imperial College London
Silwood Park
Ascot
Berks
SL5 7PY
UK
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Phone (0) 207 5942 216
Fax (0) 207 5942 339
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The data are from a bioassay in which a factorial experiment with two
factors (antibio and toxin) each with three levels was replicated four
times. The response is "mi" and "n-mi"
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Note that lines 17 to 20 in the dataframe have no failures (24 dead out
of 24 individuals)
data<-read.table("c:\\temp\\ab1.txt",header=3DT)
attach(data)
names(data)
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[1] "antibio" "toxin" "rep" "alive" "af" "dead" "mi"
[8] "n"=20=20=20=20
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data
=20
=20
=20
antibio toxin rep alive af dead mi n
1 camp control 1 24 0 0 0 24 2 camp control 2 23 0 1 1 24 3 camp control 3 23 0 1 1 24 4 camp control 4 21 0 3 3 24 5 camp Cry1Ab 1 21 4 3 7 24 6 camp Cry1Ab 2 20 3 4 7 24 7 camp Cry1Ab 3 20 4 4 8 24 8 camp Cry1Ab 4 18 7 6 13 24 9 camp Vip3A 1 7 3 17 20 2410 camp Vip3A 2 10 6 14 20 24 11 camp Vip3A 3 11 5 13 18 24 12 camp Vip3A 4 10 2 14 16 24 13 bcock control 1 21 0 3 3 24 14 bcock control 2 24 0 0 0 24 15 bcock control 3 24 3 0 3 24 16 bcock control 4 23 1 1 2 24
17 bcock Cry1Ab 1 4 4 20 24 24 18 bcock Cry1Ab 2 4 4 20 24 24 19 bcock Cry1Ab 3 1 1 23 24 24 20 bcock Cry1Ab 4 2 2 22 24 2421 bcock Vip3A 1 11 4 13 17 24 22 bcock Vip3A 2 15 5 9 14 24 23 bcock Vip3A 3 5 3 19 22 24 24 bcock Vip3A 4 10 6 14 20 24
25 ana control 1 23 0 1 1 24 26 ana control 2 23 0 1 1 24 27 ana control 3 22 0 2 2 24 28 ana control 4 23 0 1 1 24 29 ana Cry1Ab 1 18 1 6 7 24 30 ana Cry1Ab 2 17 4 7 11 24 31 ana Cry1Ab 3 13 4 11 15 24 32 ana Cry1Ab 4 14 3 10 13 24 33 ana Vip3A 1 21 12 3 15 24 34 ana Vip3A 2 12 6 12 18 24 35 ana Vip3A 3 9 5 15 20 24 36 ana Vip3A 4 9 1 15 16 24
Call:
glm(formula =3D y ~ antibio * toxin, family =3D quasibinomial)
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Deviance Residuals:=20
Min 1Q Median 3Q Max=20=20
-2.0437 -0.5645 -0.1022 0.6921 1.9996=20=20
=20
Coefficients:
Estimate Std. Error t value Pr(>|t|)=20=20=20=
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(Intercept) -2.901e+00 5.020e-01 -5.780 3.79e-06 ***
antibiobcock 5.035e-01 6.441e-01 0.782 0.441=20=20=20=
=20
antibiocamp 2.100e-15 7.099e-01 2.96e-15 1.000=20=20=20=
=20
toxinCry1Ab 2.818e+00 5.494e-01 5.129 2.15e-05 ***
toxinVip3A 3.840e+00 5.600e-01 6.857 2.29e-07 ***
antibiobcock:toxinCry1Ab 2.050e+01 2.365e+03 0.009 0.993=20=20=20=
--- Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1=20Received on Sat 03 May 2008 - 17:37:37 GMT
=20
(Dispersion parameter for quasibinomial family taken to be 1.19442)
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Null deviance: 515.115 on 35 degrees of freedom Residual deviance: 35.047 on 27 degrees of freedom AIC: NA
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Number of Fisher Scoring iterations: 17
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# note the standard error of the firast interaction term =3D 2365)
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# add a single failure to one replicate
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y2<-y y2[17,]<-c(23,1) model2<-glm(y2~antibio*toxin,quasibinomial) summary(model2)
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Call: glm(formula =3D y2 ~ antibio * toxin, family =3D quasibinomial)
=20
Deviance Residuals:=20 Min 1Q Median 3Q Max=20=20 -2.044 -0.627 -0.221 0.709 2.000=20=20
=20
Coefficients: Estimate Std. Error t value Pr(>|t|)=20=20=20=
=20
(Intercept) -2.901e+00 5.251e-01 -5.526 7.44e-06 *** antibiobcock 5.035e-01 6.737e-01 0.747 0.461=20=20=20=
=20
antibiocamp -4.058e-18 7.426e-01 -5.47e-18 1.000=20=20=20=
=20
toxinCry1Ab 2.818e+00 5.747e-01 4.904 3.94e-05 *** toxinVip3A 3.840e+00 5.857e-01 6.556 4.96e-07 *** antibiobcock:toxinCry1Ab 4.134e+00 1.352e+00 3.057 0.005 **=20 antibiocamp:toxinCry1Ab -4.721e-01 8.153e-01 -0.579 0.567=20=20=20=
=20
antibiobcock:toxinVip3A -2.868e-01 7.720e-01 -0.372 0.713=20=20=20=
=20
antibiocamp:toxinVip3A 2.748e-01 8.341e-01 0.329 0.744=20=20=20=
=20
--- Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1=20
=20
(Dispersion parameter for quasibinomial family taken to be 1.306703)
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Null deviance: 506.602 on 35 degrees of freedom Residual deviance: 37.851 on 27 degrees of freedom AIC: NA
=20
Number of Fisher Scoring iterations: 5
=20
# Now the standard errors are all well-behaved
=20
=20
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