Calculating Binomial Probabilities (DP IB Maths: AI SL)

Throughout this section we will use the random variable . For binomial, the probability of taking a non-integer or negative value is always zero. Therefore any values of  mentioned in this section will be assumed to be non-negative integers.

How do I calculate P(X = x): the probability of a single value for a binomial distribution?

• You should have a GDC that can calculate binomial probabilities
• You want to use the "Binomial Probability Distribution" function
  • This is sometimes shortened to BPD, Binomial PD or Binomial Pdf
• You will need to enter:
  • The 'x' value - the value of x for which you want to find 
  • The 'n' value - the number of trials
  • The 'p' value - the probability of success
• Some calculators will give you the option of listing the probabilities for multiple values of x at once
• There is a formula that you can use but you are expected to be able to use the distribution function on your GDC
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How do I calculate P(a ≤ X ≤ b): the cumulative probabilities for a binomial distribution? 

• You should have a GDC that can calculate cumulative binomial probabilities
  • Most calculators will find 
  • Some calculators can only find 
    • The identities below will help in this case
• You should use the "Binomial Cumulative Distribution" function
  • This is sometimes shortened to BCD, Binomial CD or Binomial Cdf
• You will need to enter:
  • The lower value - this is the value a
    • This can be zero in the case 
  • The upper value - this is the value b
    • This can be n in the case 
  • The 'n' value - the number of trials
  • The 'p' value - the probability of success

How do I find probabilities if my GDC only calculates P(X ≤ x)?

• To calculate P(X ≤ x) just enter x into the cumulative distribution function
• To calculate P(X < x) use:
  • which works when X is a binomial random variable
    • P(X < 5) = P(X ≤ 4)

• To calculate P(X > x) use:
  • which works for any random variable X 
    • P(X > 5) = 1 - P(X ≤ 5)
• To calculate P(X ≥ x) use:
  •  which works when X is a binomial random variable
    • P(X ≥ 5) = 1 - P(X ≤ 4)
• To calculate P(a ≤ X ≤ b) use:
  •  which works when X is a binomial random variable
    • P(5 ≤ X ≤ 9) = P(X ≤ 9) - P(X ≤ 4)

What if an inequality does not have the equals sign (strict inequality)? 

• For a binomial distribution (as it is discrete) you could rewrite all strict inequalities (< and >) as weak inequalities (≤ and ≥) by using the identities for a binomial distribution
  • and 
  • For example: P(X < 5) = P(X ≤ 4) and P(X > 5) = P(X ≥ 6)
• It helps to think about the range of integers you want
  • Identify the smallest and biggest integers in the range
• If your range has no minimum or maximum then use 0 or n
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  • P(5 < X ≤ 9) = P(6 ≤ X ≤ 9)
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  • P(5 ≤ X < 9) = P(5 ≤ X ≤ 8)
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  • P(5 < X < 9) = P(6 ≤ X ≤ 8)