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Marrying Tukey's HSD and ANOVA results

2 messages · Lara R. Appleby 04, Ben Bolker

#
I've done a standard two way ANOVA using glm on the dependent variable "clutchsize"  
with the two factors "treatment" (which has 3 levels called 1, 2, and 3) and "species"  
(which has two levels called 1 and 2). Apparently there is no significant interaction  
term. Then I did Tukey's HSD and found that there were significant differences  
between species at only one of the three treatment levels, treatment level 1.  
Are these in fact conflicting results?

##ANOVA RESULTS
Df Sum Sq Mean Sq F value   Pr(>F)
treatment           1  29.26  29.264  7.0230  0.00884 **
species             1 138.14 138.143 33.1526 4.13e-08 ***
treatment:species   1   8.11   8.110  1.9464  0.16487
Residuals         163 679.20   4.167
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

##TUKEY HSD RESULTS
Tukey multiple comparisons of means
     95% family-wise confidence level

Fit: aov(formula = clutchsize ~ treatment * species)

$treatment
           diff        lwr       upr     p adj
2-1  1.3245614  0.4184292 2.2306936 0.0020030
3-1  1.0416667  0.1316071 1.9517262 0.0204117
3-2 -0.2828947 -1.1806793 0.6148899 0.7368331

$species
         diff       lwr       upr p adj
2-1 -1.89988 -2.544747 -1.255013     0

$`treatment:species`
               diff         lwr        upr     p adj
2:1-1:1  1.1791506 -0.19269072  2.5509919 0.1364846
3:1-1:1  0.6476190 -0.73345324  2.0286913 0.7550479
1:2-1:1 -2.4225564 -4.08045729 -0.7646555 0.0005858
2:2-1:1 -0.8357143 -2.46652980  0.7951012 0.6787094
3:2-1:1 -0.6357143 -2.26652980  0.9951012 0.8706501
3:1-2:1 -0.5315315 -1.89354633  0.8304833 0.8701101
1:2-2:1 -3.6017070 -5.24376636 -1.9596476 0.0000000
2:2-2:1 -2.0148649 -3.62957317 -0.4001566 0.0055886
3:2-2:1 -1.8148649 -3.42957317 -0.2001566 0.0177862
1:2-3:1 -3.0701754 -4.71995456 -1.4203963 0.0000041
2:2-3:1 -1.4833333 -3.10589150  0.1392248 0.0944158
3:2-3:1 -1.2833333 -2.90589150  0.3392248 0.2077935
2:2-1:2  1.5868421 -0.27701672  3.4507009 0.1438444
3:2-1:2  1.7868421 -0.07701672  3.6507009 0.0684872
3:2-2:2  0.2000000 -1.63980803  2.0398080 0.9995894
2 days later
#
Lara R. Appleby 04 <Lara.R.Appleby.04 at ...> writes:
I think you failed to tell R that `treatment' was a factor
(i.e. a categorical variable).  The Df in the output above
suggests that you are actually (accidentally) running a linear
regression on treatment number, rather than an ANOVA on
treatment.  The Df should be 2,1,2 for the design you
described above.

  That doesn't answer your other question, but it would
be better to sort out the more fundamental issue first.