# B. Look at independence:
# i. plot residuals vs each covariate in the model
# Fish_Length
plot(x = data$Z_Length, 
     y = E1, 
     xlab = "Z Length",
     ylab = "Normalized residuals")
abline(h = 0, lty = 2)
# Note: observed groupings are created by the nature of the data because in the data set 
# we only measured individuals from 5 categories of lengths (big, small, and three groups in between)
# Species
boxplot(E1 ~ Fish_Species,   
        ylab = "Normalized residuals",
        data = data, xlab = "Species")
abline(h = 0, lty = 2)
#Lake
boxplot(E1 ~ Lake,   
        ylab = "Normalized residuals",
        data = data, xlab = "Lake")
abline(h = 0, lty = 2)