t-test: two-tail: Calculate p value properly. (#24)

This change fixes #23 as per https://support.minitab.com/en-us/minitab/18/help-and-how-to/statistics/basic-statistics/supporting-topics/basics/manually-calculate-a-p-value/

lib/statistics/statistical_test/t_test.rb

@@ -39,9 +39,16 @@ def self.perform(alpha, tails, *args)
  (sample_left_mean - sample_right_mean).abs/standard_error.to_f
  end
 
- probability = Distribution::TStudent.new(degrees_of_freedom).cumulative_function(t_score)
- p_value = 1 - probability
- p_value *= 2 if tails == :two_tail
+ t_distribution = Distribution::TStudent.new(degrees_of_freedom)
+ probability = t_distribution.cumulative_function(t_score)
+
+ # Steps grabbed from https://support.minitab.com/en-us/minitab/18/help-and-how-to/statistics/basic-statistics/supporting-topics/basics/manually-calculate-a-p-value/
+ # See https://github.com/estebanz01/ruby-statistics/issues/23
+ p_value = if tails == :two_tail
+ 2 * (1 - t_distribution.cumulative_function(t_score.abs))
+ else
+ 1 - probability
+ end
 
  { t_score: t_score,
  probability: probability,