Calculate Standard Deviation in Ruby

#!/usr/bin/ruby -w
include Math
#Data we are going to calculate stdev
arrValues = [ 4.58, 4.53, 4.1, 4.05 ]
fMedian = 0
arrValues.each do |fValue|
fMedian += fValue
end
fMedian /= arrValues.size.to_f
puts "Mittelwert = " + fMedian.to_s
fStandardDeviation = 0
arrValues.each do |fValue|
fStandardDeviation += (fValue - fMedian)**2
end
puts "Zwischensumme = " + fStandardDeviation.to_s
fStandardDeviation /= arrValues.size.to_f
puts fStandardDeviation
fStandardDeviation = Math.sqrt(fStandardDeviation)
fStandardDeviation = "%.3f" % fStandardDeviation
puts "rating = " + fStandardDeviation.to_s