class ActiveGenie::Ranking::PlayersCollection

def coefficient_of_variation
  score_list = eligible.map(&:score).compact
  return nil if score_list.empty?
  mean = score_list.sum.to_f / score_list.size
  return nil if mean == 0
  variance = score_list.map { |num| (num - mean) ** 2 }.sum / score_list.size
  standard_deviation = Math.sqrt(variance)
  (standard_deviation / mean) * 100
end