gistfile1.rb

def variance(population)
      n = 0
      mean = 0.0
      s = 0.0
      population.each { |x|
        n = n + 1
        delta = x - mean
        mean = mean + (delta / n)
        s = s + delta * (x - mean)
      }
      return s / n
    end

    def standard_deviation(population)
      Math.sqrt(variance(population))
    end
    
    def median(collection)
      collection.sort!
      middle = collection.size.to_f / 2
      return collection.first if collection.size == 1;
      a = middle.ceil
      return (collection[a-1.to_i] + collection[a.to_i]) / 2 if middle % 2
      collection[middle.to_i]
    end