# Generating Data with a Given Correlation

Using Julia to generate a dataset with a given correlation

Julia
Tutorial
Brief
Author

EE

Published

September 8, 2022

This is going to be a short one, but I saw a comment on Twitter recently about an interview question where someone was asked to generate a dataset with variables X and Y that are correlated at r = .8. So I figured I’d write out some code that does this as a way to practice in Julia a little bit more.

using Statistics
using Distributions
using CairoMakie #for plotting
using Random #to set a seed

Random.seed!(0408)
TaskLocalRNG()

The approach here is going to be to define a covariance (correlation) matrix and a vector of means, then define a multivariate normal distribution parameterized by these things. We’ll then use this distribution to generate our data.

First we’ll define $$\Sigma$$, which is our covariance matrix. Since we’re generating a dataset with only 2 variables, this will be a 2x2 matrix, where the diagonals will be 1 and the off-diagonals will be .8, which is the correlation we want between X and Y.

#define our covariance matrix
Σ = [[1.0, .8] [.8, 1.0]]
2×2 Matrix{Float64}:
1.0  0.8
0.8  1.0

Then we’ll define a mean vector. This will be a 2-element vector (one for each variable), but we don’t actually care what the values are here, so let’s just make them 0.

#define a mean vector
#we don't actually care what these values are, though
μ = zeros(2)
2-element Vector{Float64}:
0.0
0.0

Now we can define a distribution given $$\Sigma$$ and $$\mu$$

d = Distributions.MvNormal(μ, Σ)
FullNormal(
dim: 2
μ: [0.0, 0.0]
Σ: [1.0 0.8; 0.8 1.0]
)

And then we can draw a sample from this distribution

s = rand(d, 200)
2×200 Matrix{Float64}:
-1.40556   0.469524  -1.19092  -0.40408   …  -0.244792  0.874835  -0.719764
-0.595655  1.01141   -1.84189  -0.550097      0.250661  1.72269   -0.862095

To confirm this works like expected, we can plot the sample

CairoMakie.scatter(s)

It looks like a .8 correlation to me. But to do a final check, we can get the correlation matrix of our sample.

#we need to transpose the matrix from 2x200 to 200x2, hence s' instead of s
cor(s')
2×2 Matrix{Float64}:
1.0       0.769654
0.769654  1.0

Close enough. Our correlation won’t be exactly equal to .8 using this approach since we’re sampling from a distribution, but there’s really no difference (imo) between a .77 correlation and a .80 correlation.

## Citation

BibTeX citation:
@online{ekholm2022,
author = {Eric Ekholm and EE},
title = {Generating {Data} with a {Given} {Correlation}},
date = {2022-09-08},
url = {https://www.ericekholm.com/posts/cor-generate-data},
langid = {en}
}