Monte Carlo simulation to estimate American deaths from COVID-19

COVID19Deaths.scala

// Monte Carlo simulation to estimate American deaths from COVID-19.
// The probability distribution of estimated deaths is taken from the
// Good Judgment Superforecaster project.
// "How many deaths attributed to COVID-19 in the U.S. will be reported/estimated as of 31 March 2021?"
// https://goodjudgment.io/covid/dashboard/

type Deaths = Int

def simulateEstimatedDeaths: Deaths = {
  import scala.util.Random

  // number returns a random number between from and to, both inclusive.
  def number(rand: Random, from: Int, to: Int): Int = {
    if (from > to) from
    else rand.nextInt(to - from + 1) + 1
  }

  val rand = new Random(System.currentTimeMillis())

  val iterations = 1000
  val placeholder = 0
  val startingDeaths = 0.0f

  Math.round(
    Array
      .fill(iterations)(placeholder)
      .foldLeft(startingDeaths)((deaths, _) => {
        deaths + (
          rand.nextFloat match {
            // These probabilities and death ranges correspond to values from
            // "How many deaths attributed to COVID-19 in the U.S. will be reported/estimated as of 31 March 2021?"
            // https://goodjudgment.io/covid/dashboard/
            case x if x < 0.06 =>
              number(rand, 8162, 35000) // 8,162 Americans died from COVID-19 as of 2020/4/4
            case x if x < 0.75 => number(rand, 35000, 350000)

            // A naive approach would draw from a uniform distribution in each bucket.
            // This yields an estimate of about 500k American deaths.
            // But it seems incorrect to draw uniformly from [350k, 3.5m] because no
            // superforecasters estimated that over 3.5m Americans would die from COVID-19,
            // so it may be the case that 99% of Superforecasters estimate that less than
            // 1m Americans will die. We don't know. Instead we'll use the lower bound of 350k.
            //case _ => number(rand, 350000, 3500000) // potentially overly pessimistic
            case _ => 350000
          }
        )
      }) / iterations
  )
}

val estimatedDeaths = simulateEstimatedDeaths
println(s"""
A Monte Carlo simulation estimates that ${java.text.NumberFormat.getIntegerInstance
  .format(estimatedDeaths)} Americans will die from COVID-19.
  
The probability distribution of estimated deaths is taken from the Good Judgment
Superforecaster project.
  
goodjudgment.io/covid/dashboard/
""")