Introduction

student t-test is used to compare the mean of two groups of samples. The aim of this article is to show how to perform a matrix of t-test in R. In this case a t-test is computed between each variable and the others. The result is a matrix of p-value containing the significance levels between all possible pairs of variables.

Custom function for multiple t-test

We will use a the following custom R function :

# matrix of t-test
# mat : data.frame or matrix
# ... : further arguments to pass to the t.test function
multi.ttest <- function(mat, ...) {
  mat <- as.matrix(mat)
  n <- ncol(mat)
  p.mat<- matrix(NA, n, n)
  diag(p.mat) <- 1
  for (i in 1:(n - 1)) {
    for (j in (i + 1):n) {
      test <- t.test(mat[, i], mat[, j], ...)
      p.mat[i, j] <- p.mat[j, i] <- test$p.value
    }
  }
  colnames(p.mat) <- rownames(p.mat) <- colnames(mat)
  signif(p.mat,4)
}

Data for t-test

The mtcars data is used in the following examples :

head(mtcars)

                   mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

We will use the first seven columns for the t-test.

Compute the matrix of t-test

p.mat<-multi.ttest(mtcars[,1:7])
p.mat

           mpg       cyl      disp        hp      drat        wt      qsec
mpg  1.000e+00 9.508e-15 7.978e-11 1.030e-11 3.164e-16 1.028e-16 5.107e-02
cyl  9.508e-15 1.000e+00 1.774e-11 8.322e-13 2.280e-09 9.118e-11 3.731e-35
disp 7.978e-11 1.774e-11 1.000e+00 1.546e-03 1.347e-11 1.293e-11 6.336e-11
hp   1.030e-11 8.322e-13 1.546e-03 1.000e+00 5.275e-13 4.919e-13 7.245e-12
drat 3.164e-16 2.280e-09 1.347e-11 5.275e-13 1.000e+00 6.025e-02 5.915e-33
wt   1.028e-16 9.118e-11 1.293e-11 4.919e-13 6.025e-02 1.000e+00 7.269e-39
qsec 5.107e-02 3.731e-35 6.336e-11 7.245e-12 5.915e-33 7.269e-39 1.000e+00

The result is a matrix of p-value between all possible pairs of variables.

Keep only the lower triangle of the matrix

The R function lower.tri() is used to hide the lower triangle of the matrix of the p-value

# Hide upper triangle
upper<-p.mat
upper[upper.tri(p.mat)]<-""
upper<-as.data.frame(upper)
upper

           mpg       cyl      disp        hp      drat        wt qsec
mpg          1                                                       
cyl  9.508e-15         1                                             
disp 7.978e-11 1.774e-11         1                                   
hp    1.03e-11 8.322e-13  0.001546         1                         
drat 3.164e-16  2.28e-09 1.347e-11 5.275e-13         1               
wt   1.028e-16 9.118e-11 1.293e-11 4.919e-13   0.06025         1     
qsec   0.05107 3.731e-35 6.336e-11 7.245e-12 5.915e-33 7.269e-39    1

Infos

This analysis has been performed using R (ver. 3.1.0).

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