Estimation of class prior probabilities by EM algorithm

A simple procedure to improve the estimation of class prior probabilities when the training data does not reflect the true a priori probabilities of the target classes. The EM algorithm used is described in Saerens et al (2002).

Usage

classPriorProbs(object, newdata = object$data, 
                itmax = 1e3, eps = sqrt(.Machine$double.eps))

Arguments

object: an object of class 'MclustDA' resulting from a call to MclustDA.
newdata: a data frame or matrix giving the data. If missing the train data obtained from the call to MclustDA are used.
itmax: an integer value specifying the maximal number of EM iterations.
eps: a scalar specifying the tolerance associated with deciding when to terminate the EM iterations.

Details

The estimation procedure employes an EM algorithm as described in Saerens et al (2002).

Value

A vector of class prior estimates which can then be used in the predict.MclustDA to improve predictions.

References

Saerens, M., Latinne, P. and Decaestecker, C. (2002) Adjusting the outputs of a classifier to new a priori probabilities: a simple procedure, Neural computation, 14 (1), 21--41.

Examples

# \donttest{
# generate data from a mixture f(x) = 0.9 * N(0,1) + 0.1 * N(3,1)
n <- 10000
mixpro <- c(0.9, 0.1)
class <- factor(sample(0:1, size = n, prob = mixpro, replace = TRUE))
x <- ifelse(class == 1, rnorm(n, mean = 3, sd = 1), 
                        rnorm(n, mean = 0, sd = 1))

hist(x[class==0], breaks = 11, xlim = range(x), main = "", xlab = "x", 
     col = adjustcolor("dodgerblue2", alpha.f = 0.5), border = "white")
hist(x[class==1], breaks = 11, add = TRUE,
     col = adjustcolor("red3", alpha.f = 0.5), border = "white")
box()


# generate training data from a balanced case-control sample, i.e.
# f(x) = 0.5 * N(0,1) + 0.5 * N(3,1)
n_train <- 1000
class_train <- factor(sample(0:1, size = n_train, prob = c(0.5, 0.5), replace = TRUE))
x_train <- ifelse(class_train == 1, rnorm(n_train, mean = 3, sd = 1), 
                                    rnorm(n_train, mean = 0, sd = 1))

hist(x_train[class_train==0], breaks = 11, xlim = range(x_train), 
     main = "", xlab = "x", 
     col = adjustcolor("dodgerblue2", alpha.f = 0.5), border = "white")
hist(x_train[class_train==1], breaks = 11, add = TRUE,
     col = adjustcolor("red3", alpha.f = 0.5), border = "white")
box()


# fit a MclustDA model
mod <- MclustDA(x_train, class_train)
summary(mod, parameters = TRUE)
#> ------------------------------------------------ 
#> Gaussian finite mixture model for classification 
#> ------------------------------------------------ 
#> 
#> MclustDA model summary: 
#> 
#>  log-likelihood    n df       BIC
#>       -1942.376 1000  4 -3912.383
#>        
#> Classes   n    % Model G
#>       0 524 52.4     X 1
#>       1 476 47.6     X 1
#> 
#> Class prior probabilities:
#>     0     1 
#> 0.524 0.476 
#> 
#> Class = 0
#> 
#> Mixing probabilities: 1 
#> 
#> Means:
#> [1] -0.03369754
#> 
#> Variances:
#> [1] 0.9532879
#> 
#> Class = 1
#> 
#> Mixing probabilities: 1 
#> 
#> Means:
#> [1] 2.955709
#> 
#> Variances:
#> [1] 1.059664
#> 
#> Training confusion matrix:
#>      Predicted
#> Class   0   1
#>     0 488  36
#>     1  29 447
#> Classification error = 0.065 
#> Brier score          = 0.0492 

# test set performance
pred <- predict(mod, newdata = x)
classError(pred$classification, class)$error
#> [1] 0.0688
BrierScore(pred$z, class)
#> [1] 0.05133817

# compute performance over a grid of prior probs
priorProp <- seq(0.01, 0.99, by = 0.01)
CE <- BS <- rep(as.double(NA), length(priorProp))
for(i in seq(priorProp))
{
  pred <- predict(mod, newdata = x, prop = c(1-priorProp[i], priorProp[i]))
  CE[i] <- classError(pred$classification, class = class)$error
  BS[i] <- BrierScore(pred$z, class)
}

# estimate the optimal class prior probs
(priorProbs <- classPriorProbs(mod, x))
#>         0         1 
#> 0.8852142 0.1147858 
pred <- predict(mod, newdata = x, prop = priorProbs)
# compute performance at the estimated class prior probs
classError(pred$classification, class = class)$error
#> [1] 0.0352
BrierScore(pred$z, class)
#> [1] 0.02658523

matplot(priorProp, cbind(CE,BS), type = "l", lty = 1, lwd = 2,
        xlab = "Class prior probability", ylab = "", ylim = c(0,max(CE,BS)), 
        panel.first = 
          { abline(h = seq(0,1,by=0.05), col = "grey", lty = 3)
            abline(v = seq(0,1,by=0.05), col = "grey", lty = 3) 
          })
abline(v = mod$prop[2], lty = 2)              # training prop
abline(v = mean(class==1), lty = 4)           # test prop (usually unknown) 
abline(v = priorProbs[2], lty = 3, lwd = 2)      # estimated prior probs
legend("topleft", legend = c("ClassError", "BrierScore"),
       col = 1:2, lty = 1, lwd = 2, inset = 0.02)


# Summary of results:
priorProp[which.min(CE)] # best prior of class 1 according to classification error
#> [1] 0.11
priorProp[which.min(BS)] # best prior of class 1 according to Brier score
#> [1] 0.09
priorProbs               # optimal estimated class prior probabilities
#>         0         1 
#> 0.8852142 0.1147858 
# }