confidenceInterval <- function(x, confidenceLevel=0.95){
    N <- length(x)
    alpha <- 1-confidenceLevel
    if (N<30)
       stat <- qt(1-alpha/2, N-1)
    else
       stat <- qnorm(1-alpha/2)
    interval <- stat * sd(x) / sqrt(N)
    mean_value <- mean(x)
    result <- c(mean_value-interval, mean_value+interval)
    names(result) <- c("lower", "upper")
    result
}