[R] Interpreting Q-Q Plots

[S Ellison]

>    And since I don't have the experience, the only way to gain it is by
>  learning from those with practice reading chicken entrails.
This can be hard on the chicken population.

Try comparing QQ plots for simulated random data from different distributions with something more immediately interpretable on the measurement scale,  such as dot plots, box plots and density plots. That should add up to a fair bit of experience quite quickly. 

#Example
par(mfrow=c(1,2))
qqnorm(x<-rlnorm(200, 1,0.5))
qqline(x)
plot(density(x))

qqnorm(x<-rnorm(200, sample(c(0,4), 200, replace=TRUE))) #bimodal
qqline(x)
plot(density(x))


and so on.

Notice that qqnorm's vertical scale by defult corresponds to the horizontal scale in density plots and stripcharts. I personally prefer datax=TRUE, but really that's only a choice about whether to face north or east when reading the entrails.

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