Dear R-users,
I am currently trying to switch from SAS to R, and am not very familiar with R
yet, so forgive me if this question is irrelevant.
If I try to find the significance of the fixed factor "spikes" in a generalized
linear mixed model, with "site" nested within "zone" as a random factor, I
compare following two models with the anova function:
model1<-lmer(aphids~spikes+(1|zone:site), method="ML", family=quasipoisson)
model2<-lmer(aphids~(1|zone:site), method="ML", family=quasipoisson)
anova(model1,model2)
This gives me a p< 2.2e-16 ***, concluding that "spikes" has a highly
significant effect on "aphids". However, when I look at the summary of model1:
summary(model1)
I find a t-value for "spikes" of -0.1166 which is really insignificant...
When I try model1 in SAS with proc glimmix, corrected for overdispersion with
"random _residual_", it also gives a p-value for "spikes" of 0,985. So if
"spikes" is not having a significant effect on "aphids", then why the above
mentionned p-value generated by anova in R?
Can anyone explain this?
Please find the dataset in the attachment.
Many thanks beforehand,
Martijn.
--
Martijn Vandegehuchte
Ghent University
Department Biology
Terrestrial Ecology Unit
K.L.Ledeganckstraat 35
B-9000 Ghent
telephone: +32 (0)9/264 50 84
e-mail: [EMAIL PROTECTED]
website TEREC: www.ecology.ugent.be/terec
site zone height spikes diameter totmas densroot
vitality aphids
C inland 74 0 43 118.9 2.2 0.730025231 0
C inland 78 26 55 143.1 1.7 0.621942697 2
C inland 117 15 118 423.4 2.8 0.763705104 0
C inland 96 0 21 291.9 10.3 0.87529976 1
C inland 91 4 85 175.3 3.9 0.8141542 0
C inland 101 8 65 254.5 1.7 0.761886051 1
C inland 85 160 300 89.4 1.5 0.488188976 0
C inland 77 0 12 56.9 2.3 0.876977153 8
C inland 59 0 13 23.3 0.4 0.991416309 23
C inland 146 44 213 216.5 2 0.348127601 0
C inland 44 0 29 79.9 4.9 0.819230769 0
C inland 113 12 250 321 0.6 0.48128509 0
C inland 64 0 35 104 1.5 0.350100604 0
C inland 64 0 41 21.7 1.9 0.783410138 2
C inland 105 12 270 74.3 1.1 0.951547779 3
C inland 105 12 270 374.7 1 0.742727515 1
C inland 86 0 65 155.9 4.9 0.885923949 17
C inland 81 0 250 97.3 0.4 0.324029126 0
C inland 83 64 379 98.6 1 0.90872211 1
C inland 116 18 250 259.5 1.6 0.426848249 0
C coast 94 0 36 257.5 5.9 0.782912621 7
C coast 121 1 120 173 1.4 0.450289017 0
C coast 73 0 8 26.1 6.2 0.831417625 43
C coast 86 0 10 74.5 1.6 0.923489933 13
C coast 98 0 140 163 1.4 0.479754601 2
C coast 488 0 15 30 8 0.47 3
C coast 80 0 51 116.2 3.2 0.697934596 4
C coast 97 15 220 241 7.1 0.676348548 0
C coast 100 3 20 153.2 1.2 0.598525469 0
C coast 84 0 24 213.5 16.5 0.914473684 0
C coast 91 0 45 308.4 5.2 0.808205796 10
C coast 79 0 150 255.1 7.5 0.483586869 3
C coast 60 0 11 28.8 2.4 0.822916667 33
C coast 91 0 100 176.5 8.7 0.956373938 17
C coast 107 3 150 536.6 6.9 0.823332091 5
C coast 65 0 41 89.3 3.9 0.909294513 9
C coast 135 38 65 864.8 10.8 0.804088785 1
C coast 87 0 47 225.1 2.4 0.966681475 13
C coast 71 0 12 28.8 2.4 0.791666667 6
C coast 92 0 500 172.5 0.8 0.408921933 0
B coast 102 0 103 201.2 0.3 0.4714809 2
B coast 99 0 100 286.6 3.4 0.941702432 18
B coast 80 63 461 85 0.8 0.5250501 6
B coast 70 0 20 77.6 4.3 0.948320413 71
B coast 79 1 120 165.2 4 0.485753053 7
B coast 64 0 28 65.5 10.9 0.948091603 32
B coast 116 45 92 164.1 1.1 0.398279041 0
B coast 76 0 23 42.5 0.8 0.65914787 2
B coast 109 49 250 190.5 1 0.421274355 3
B coast 79 0 49 302.9 11.2 0.902938263 49
B coast 81 0 150 112.4 1.9 0.35387674 1
B coast 80 4 20 101.7 2.7 0.438398357 0
B coast 83 0 20 427.5 47.8 0.85754386 16
B coast 67 0 2 12.7 1.2 0.818897638 46
B coast 91 10 37 369.2 9.5 0.653543307 7
B coast 98 3 66 321 5.2 0.629283489 6
B coast 68 0 30 185.8 17.9 0.75780409 107
B coast 82 0 136 241.8 1.9 0.836546521 14
B coast 114 3 60 388.2 18.5 0.775752773 38
B coast 58 0 21 154.5 32 0.963106796 177
A inland 105 0 200 320 1 0.225776105 0
A inland 155 0 79 104.9 0.7 0.28343949 2
A inland 70 0 5 15.9 1 0.8 0
A inland 120 2 128 609.6 1.6 0.41930144 0
A inland 103 5 40 142.3 0.5 0.284328358 0
A inland 80 0 41 39.2 0.5 0.321621622 1
A inland 110 4 300 184.5 0.1 0.597005988 7
A inland 116 0 143 388.1 14.5 0.819376449 9
A inland 77 0 30 37.2 2.8 0.774193548 13
A inland 106 0 145 277.8 1.8 0.524972658 1
A inland 82 0 44 173.6 4.5 0.727056019 0
A inland 65 0 4 18.9 4.8 0.873015873 9
A inland 126 89 160 151.9 0.7 0.428759894 6
A inland 79 9 79 124.5 1 0.64497992 1
A inland 92 0 15 215.6 10.6 0.760982522 2
A inland 123 1 180 552 7.4 0.58513146 1
A inland 102 0 63 379.3 4.4 0.668081719 1
A inland 88 0 56 268.7 5 0.819376643 6
A inland 132 99 310 198.1 2.5 0.464411913 0
A inland 51 0 6 18.9 0.6 0.947089947 1
B inland 103 0 70 314.8 1.7 0.793837357 1
B inland 580 18 128 385.6 2 0.603219943 1
B inland 74 0 16 39.5 1.4 0.637974684 0
B inland 108 0 265 414.6 5.2 0.46092233 1
B inland 121 0 265 270.5 2.1 0.152793193 0
B inland 92 0 28 135.3 5.3 0.492378049 2
B inland 116 356 500 318 2.9 0.232075472 1
B inland 77 0 41 46.2 4.1 0.876623377 2
B inland 280 0 124 474.7 1.9 0.584369075 6
B inland 103 0 31 502.3 12.6 0.692215807 1
B inland 81 0 7 43.8 3.7 0.80778032 187
B inland 58 0 46 118.1 37.2 0.876375953 489
B inland 132 61 141 519 1.1 0.620289855 6
B inland 116 1 102 442.9 17.2 0.641905622 5
B inland 89 4 79 95.6 4.5 0.324590164 1
B inland 129 122 180 535.2 6.7 0.604885615 2
B inland 111 5 102 473.2 7.6 0.742930591 1
B inland 81 0 12 36.7 2.5 0.795640327 2
B inland 96 2 150 297.1 7 0.726594301 1
B inland 104 5 119 221 2.3 0.658823529 3
A coast 67 0 49 199.3 5.4 0.486486486 1
A coast 93 11 220 291.7 3.9 0.573041894 15
A coast 83 10 61 103.5 1.7 0.767597765 2
A coast 93 2 21 171.9 0.6 0.885155773 10
A coast 67 0 37 71.7 1.8 0.804843305 70
A coast 90 4 79 171.5 4.2 0.486038394 37
A coast 85 0 9 94.4 4.1 0.424033149 10
A coast 85 6 126 120 1.7 0.714285714 22
A coast 96 9 25 156.4 3.2 0.391165173 3
A coast 110 0 79 215.6 5.3 0.354575946 0
A coast 90 1 62 187.6 0.1 0.773998903 3
A coast 84 0 14 19.9 1.8 0.729166667 0
A coast 100 2 160 190.7 4 0.70443613 26
A coast 105 28 92 285.9 1.4 0.385240113 0
A coast 95 0 60 306.3 5.1 0.743715312 2
A coast 49 0 27 31.2 1.6 0.440514469 0
A coast 87 1 42 186.7 6.6 0.549257017 0
A coast 79 1 124 262.8 6.4 0.331430746 0
A coast 73 1 50 78 2.3 0.469717362 0
A coast 101 4 300 219.1 1.9 0.37628866 0
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