5.29.2005

Q&A: What's the expected number of correct answers...

...in this interview exam?

A: mean = np = 5 x 0.25 = 1.25, with a standard deviation of = (n*p*(1 - p)) ^0.5 = (5 * 0.25 * 0.75)^0.5 = 0.47.

  • can NOT approximate the binomial with a normal distribution: approximation works best if np >= 5 and n(1-p) >= 5.

Bonus Points

  • The normal approximation to the binomial is useful whenever the binomial tables cannot be used (ie. the number of trials is so large).
  • Since the normal distribution is symmetrical, it best approximates binomial distributions that are reasonably symmetrical (ie. p close to 0.5).

Category: Quantitative Analysis > Probability > Distribution

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