How can I do standard deviation in Ruby?

Question

I have several records with a given attribute, and I want to find the standard deviation.

How do I do that?

Answer 1

If the records at hand are of type Integer or Rational, you may want to compute the variance using Rational instead of Float to avoid errors introduced by rounding.

For example:

def variance(list)
  mean = list.reduce(:+)/list.length.to_r
  sum_of_squared_differences = list.map { |i| (i - mean)**2 }.reduce(:+)
  sum_of_squared_differences/list.length
end

(It would be prudent to add special-case handling for empty lists and other edge cases.)

Then the square root can be defined as:

def std_dev(list)
  Math.sqrt(variance(list))
end