don't use anova(), use car::Anova()

anova-bad-Anova-good.R

# anova() uses type I SS and unless you have only categorical predictors and
# balanced sample sizes, the order of the formula will change p-values,
# sometimes drastically!

m1 <- lm(Volume ~ Height + Girth, data = trees)
m2 <- lm(Volume ~ Girth + Height, data = trees)

anova(m1)
#> Analysis of Variance Table
#> 
#> Response: Volume
#>           Df Sum Sq Mean Sq F value    Pr(>F)    
#> Height     1 2901.2  2901.2  192.53 4.503e-14 ***
#> Girth      1 4783.0  4783.0  317.41 < 2.2e-16 ***
#> Residuals 28  421.9    15.1                      
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
anova(m2)
#> Analysis of Variance Table
#> 
#> Response: Volume
#>           Df Sum Sq Mean Sq  F value  Pr(>F)    
#> Girth      1 7581.8  7581.8 503.1503 < 2e-16 ***
#> Height     1  102.4   102.4   6.7943 0.01449 *  
#> Residuals 28  421.9    15.1                     
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

# This is almost never what you want. Just don't use anova()!


# Anova() from the car package, does type II SS by default—a marginal hypothesis
# test.  The order of the formula doesn't matter.

library(car)
#> Loading required package: carData
Anova(m1)
#> Anova Table (Type II tests)
#> 
#> Response: Volume
#>           Sum Sq Df  F value  Pr(>F)    
#> Height     102.4  1   6.7943 0.01449 *  
#> Girth     4783.0  1 317.4129 < 2e-16 ***
#> Residuals  421.9 28                     
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Anova(m2)
#> Anova Table (Type II tests)
#> 
#> Response: Volume
#>           Sum Sq Df  F value  Pr(>F)    
#> Girth     4783.0  1 317.4129 < 2e-16 ***
#> Height     102.4  1   6.7943 0.01449 *  
#> Residuals  421.9 28                     
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

# This is almost always what you want (unless your model object doesn't have an
# Anova() method, in which case the anova() method *might* work correctly—check
# the help file!)