Simple Monte Carlo to predict who will control US Congress after the 2016 elections

congress-carlo.rb

#! /usr/bin/env ruby

class CongressCarlo

  TRIALS = 100_000
  STAR_SCALE = 500

  attr_reader :name, :dems_not_running, :reps_not_running

  def estimate(*ratings)
    ratings.inject(0) { |t, e| t + e } / ratings.size             # arithmetic
    # ratings.inject(1.0) { |t, e| t * e } ** (1.0 / ratings.size)  # geometric
  end

  def safe_d;   0.9; end
  def likely_d; 0.8; end
  def lean_d;   0.7; end
  def tilt_d;   0.6; end
  def tossup;   0.5; end
  def tilt_r;   1.0 - tilt_d; end
  def lean_r;   1.0 - lean_d; end
  def likely_r; 1.0 - likely_d; end
  def safe_r;   1.0 - safe_d; end

  def odds_of_dem_win_per_race
    raise 'override me'
  end

  def run
    # Run the helper method onces
    races = odds_of_dem_win_per_race

    trials_by_dem_win = [0] * races.size
    TRIALS.times do
      dem_wins = 0

      races.each do |odds_of_dem_win|
        if rand() < odds_of_dem_win
          dem_wins += 1
        end
      end

      trials_by_dem_win[dem_wins] += 1
    end

    puts "Odds of Democratic seats in the #{name}:"
    puts

    majority = (0.5 * (dems_not_running + races.size + reps_not_running)).ceil
    trials_with_rep_super_majority = 0
    trials_with_dem_majority = 0
    trials_with_dem_super_majority = 0

    percentiles = []
    trials_seen = 0

    trials_by_dem_win.each_with_index do |trials, dem_wins|
      elected_dems = dems_not_running + dem_wins

      trials_seen += trials
      lower_percentile = percentiles.size
      upper_percentile = ((100.0 * trials_seen) / TRIALS).ceil
      while percentiles.size < upper_percentile
        percentiles << elected_dems
      end

      trials_with_rep_super_majority += trials if elected_dems < majority
      trials_with_dem_majority += trials if elected_dems >= majority
      trials_with_dem_super_majority += trials if elected_dems > majority

      stars = '*' * (STAR_SCALE * (1.0 * trials) / TRIALS).round
      next if stars == ''
      m = elected_dems == majority ? 'M' : ' '
      puts "#{m} #{elected_dems} (#{lower_percentile.to_s.rjust(3)}): #{stars}"
    end

    odds_of_rep_super_majority = (1.0 * trials_with_rep_super_majority) / TRIALS
    odds_of_dem_majority = (1.0 * trials_with_dem_majority) / TRIALS
    odds_of_dem_super_majority = (1.0 * trials_with_dem_super_majority) / TRIALS

    puts
    puts "Democratic seats at 90% confidence: #{percentiles[10]}"
    puts "Democratic seats at 75% confidence: #{percentiles[25]}"
    puts "Democratic seats at 50% confidence: #{percentiles[50]}"

    puts
    puts "Republican super majority: #{(100 * odds_of_rep_super_majority).round(1)}%"
    puts "Democratic       majority: #{(100 * odds_of_dem_majority).round(1)}%"
    puts "Democratic super majority: #{(100 * odds_of_dem_super_majority).round(1)}%"
  end

end

# https://en.wikipedia.org/wiki/United_States_Senate_elections,_2016
class Senate < CongressCarlo

  def initialize
    @name = 'Senate'
    @dems_not_running = 34 + 2  # 2 independents caucus with democrats
    @reps_not_running = 30
  end

  def odds_of_dem_win_per_race
    [estimate(safe_d, safe_d, safe_d, safe_d)] * 7 +
    [
      estimate(likely_r,  safe_r,   safe_r,   safe_r),    # Alaska
      estimate(likely_r,  lean_r,   lean_r,   lean_r),    # Arizona
      estimate(likely_d,  safe_d,   safe_d,   safe_d),    # California
      estimate(lean_d,    lean_d,   lean_d,   lean_d),    # Colorado
      estimate(tossup,    tossup,   tossup,   tossup),    # Florida
      estimate(likely_r,  safe_r,   likely_r, safe_r),    # Georgia
      estimate(tossup,    lean_d,   tilt_d,   tossup),    # Illinois
      estimate(likely_r,  likely_r, likely_r, likely_r),  # Indiana
      estimate(safe_r,    likely_r, safe_r,   safe_r),    # Kentucky
      estimate(safe_r,    likely_r, safe_r,   safe_r),    # Louisiana
      estimate(likely_r,  likely_r, safe_r,   likely_r),  # Missouri
      estimate(tossup,    tossup,   tossup,   tossup),    # Nevada
      estimate(tossup,    tossup,   tilt_r,   tossup),    # New Hampshire
      estimate(lean_r,    likely_r, lean_r,   likely_r),  # North Carolina
      estimate(lean_r,    lean_r,   lean_r,   lean_r),    # Ohio
      estimate(lean_d,    lean_d,   tilt_d,   lean_d),    # Pennsylvania
      estimate(tossup,    lean_d,   tossup,   tossup),    # Wisconsin
    ] +
    [estimate(safe_r, safe_r, safe_r, safe_r)] * 10
  end

end

# https://en.wikipedia.org/wiki/United_States_House_of_Representatives_elections,_2016
class House < CongressCarlo

  def initialize
    @name = 'House'
    @dems_not_running = 0
    @reps_not_running = 0
  end

  def odds_of_dem_win_per_race
    [estimate(safe_d, safe_d, safe_d, safe_d)] * 166 +
    [
      estimate(safe_r,    safe_r,   likely_r),  # AK AL
      estimate(tossup,    tossup,   tossup),    # AZ 1
      estimate(lean_r,    lean_r,   tossup),    # AZ 2
      estimate(likely_d,  safe_d,   likely_d),  # AZ 9
      estimate(lean_d,    likely_d, lean_d),    # CA 7
      estimate(lean_r,    safe_r,   lean_r),    # CA 10
      estimate(likely_d,  safe_d,   likely_d),  # CA 16
      estimate(lean_r,    likely_r, lean_r),    # CA 21
      estimate(likely_d,  safe_d,   lean_d),    # CA 24
      estimate(lean_r,    lean_r,   likely_r),  # CA 25
      estimate(safe_d,    safe_d,   likely_d),  # CA 26
      estimate(likely_d,  safe_d,   likely_d),  # CA 31
      estimate(likely_d,  safe_d,   likely_d),  # CA 36
      estimate(likely_d,  safe_d,   lean_d),    # CA 52
      estimate(tossup,    lean_r,   tossup),    # CO 6
      estimate(likely_d,  safe_d,   safe_d),    # CT 5
      estimate(lean_r,    likely_d, lean_d),    # FL 2
      estimate(likely_r,  safe_r,   safe_r),    # FL 7
      estimate(lean_d,    safe_r,   safe_r),    # FL 10
      estimate(lean_d,    likely_r, tossup),    # FL 13
      estimate(tossup,    tossup,   tossup),    # FL 18
      estimate(tossup,    tilt_r,   tossup),    # FL 26
      estimate(tossup,    tossup,   tossup),    # IL 10
      estimate(likely_r,  likely_r, likely_r),  # IL 12
      estimate(likely_r,  safe_r,   likely_r),  # IL 13
      estimate(safe_r,    safe_r,   safe_r),    # IN 2
      estimate(tossup,    tilt_d,   lean_d),    # IA 1
      estimate(likely_d,  safe_d,   likely_d),  # IA 2
      estimate(lean_r,    tossup,   lean_r),    # IA 3
      estimate(tossup,    tossup,   tossup),    # ME 2
      estimate(likely_d,  safe_d,   likely_d),  # MD 6
      estimate(lean_r,    lean_r,   tossup),    # MI 1
      estimate(lean_r,    tilt_r,   lean_r),    # MI 7
      estimate(likely_r,  safe_r,   likely_r),  # MI 8
      estimate(safe_d,    safe_d,   likely_d),  # MN 1
      estimate(tossup,    tossup,   tossup),    # MN 2
      estimate(likely_d,  safe_d,   likely_d),  # MN 7
      estimate(lean_d,    likely_d, lean_d),    # MN 8
      estimate(tossup,    tossup,   tossup),    # NE 2
      estimate(tossup,    tossup,   lean_r),    # NV 3
      estimate(lean_d,    tilt_d,   lean_d),    # NV 4
      estimate(tossup,    tossup,   tossup),    # NH 1
      estimate(safe_d,    safe_d,   likely_d),  # NH 2
      estimate(likely_r,  safe_r,   likely_r),  # NJ 3
      estimate(lean_r,    likely_r, likely_r),  # NJ 5
      estimate(likely_r,  safe_r,   likely_r),  # NM 2
      estimate(lean_r,    tilt_r,   tossup),    # NY 1
      estimate(safe_d,    safe_d,   safe_d),    # NY 4
      estimate(likely_r,  safe_r,   likely_r),  # NY 11
      estimate(likely_d,  safe_d,   likely_d),  # NY 18
      estimate(tossup,    tossup,   tossup),    # NY 19
      estimate(likely_r,  safe_r,   likely_r),  # NY 21
      estimate(likely_r,  safe_r,   likely_r),  # NY 23
      estimate(lean_r,    tossup,   tossup),    # NY 24
      estimate(likely_d,  safe_d,   likely_d),  # NY 25
      estimate(likely_r,  safe_r,   safe_r),    # NC 9
      estimate(safe_r,    safe_r,   likely_r),  # OH 14
      estimate(likely_r,  likely_r, likely_r),  # PA 6
      estimate(tossup,    tilt_r,   tossup),    # PA 8
      estimate(tossup,    tossup,   tossup),    # TX 23
      estimate(likely_r,  safe_r,   likely_r),  # UT 4
      estimate(safe_r,    safe_r,   likely_r),  # VA 2
      estimate(lean_r,    safe_r,   likely_r),  # VA 4
      estimate(likely_r,  lean_r,   likely_r),  # VA 10
      estimate(likely_r,  safe_r,   safe_r)     # WV 2
    ] +
    [estimate(safe_r, safe_r, safe_r, safe_r)] * 204
  end

end

Senate.new.run
puts
House.new.run