Question

Im writing some code in R whereby i need to generate a data set from specific criteria given they meet certain conditions.

I have three probabilities
A – 0.423, B – 0.324 and C- 0.253.

I want to run a random generated sample runif(50, 0, 1).

If the number generated lies between 0 and 0.423, i want to generate a value from rnorm(50, 25, 4),
if its between 0.423 and 0.747, i want to generate a value from rnorm(50, 28, 4.5)
and finally if its between 0.747 and 1, i want to generate a value from rnorm(50, 30, 5).

I was trying to do this using some sort of compound ifelse function but to no avail.

Any suggestions?

Cheers

Answer 1

As you suspected, this can be done using ifelse:

u = runif(50, 0, 1)
values = ifelse(u < .423, rnorm(50, 25, 4),
                ifelse(u < .747, rnorm(50, 28, 4.5),
                       rnorm(50, 30, 5)))

This works because ifelse can operate along vectors.

You might prefer this method, especially if you end up having more than three sections to break it into:

means = c(25, 28, 30)
vars = c(4, 4.5, 5)
probs = c(.423, .324, .253)
samples = sample(1:3, 50, replace=TRUE, prob=probs)
values = rnorm(50, means[samples], vars[samples])