Math Problem Statement

41% of people say yes and 49% say no out of the whole population. You randomly sample 100 people, what's the probability that a majority of them say yes?

Solution

To solve this, we need to compute the probability that a majority of the sampled 100 people say "yes". This means we want the number of people who say "yes" to be greater than 50 out of the 100.

Step-by-Step Approach:

  1. Define the Problem:

    • Probability of a single person saying "yes" is p=0.41p = 0.41.
    • Probability of a single person saying "no" is 1p=0.591 - p = 0.59.
    • We sample 100 people, so the total number of people in the sample is n=100n = 100.
    • We want the probability that more than 50 people say "yes", i.e., we need X>50X > 50, where XX is the number of people saying "yes".
  2. Use the Binomial Distribution: Since each person's response is independent, the number of "yes" responses follows a binomial distribution: XBinomial(n=100,p=0.41)X \sim \text{Binomial}(n = 100, p = 0.41) The binomial distribution has the following probability mass function (PMF): P(X=k)=(nk)pk(1p)nkP(X = k) = \binom{n}{k} p^k (1 - p)^{n - k} However, calculating this probability directly for all values of X>50X > 50 can be cumbersome. So, we use an approximation.

  3. Normal Approximation to the Binomial Distribution: The binomial distribution can be approximated by a normal distribution when nn is large, and this approximation works well here because n=100n = 100 is sufficiently large.

    The parameters for the normal distribution are:

    • Mean (μ\mu): μ=np=100×0.41=41\mu = np = 100 \times 0.41 = 41
    • Variance (σ2\sigma^2): σ2=np(1p)=100×0.41×0.59=24.19\sigma^2 = np(1 - p) = 100 \times 0.41 \times 0.59 = 24.19
    • Standard deviation (σ\sigma): σ=24.194.92\sigma = \sqrt{24.19} \approx 4.92

    So, XX can be approximated by: XN(μ=41,σ=4.92)X \sim \mathcal{N}(\mu = 41, \sigma = 4.92)

  4. Apply the Normal Approximation: To find the probability that more than 50 people say "yes", we need to compute: P(X>50)P(X > 50) First, we standardize this to a Z-score using: Z=XμσZ = \frac{X - \mu}{\sigma} For X=50X = 50, we compute the Z-score: Z=50414.92=94.921.83Z = \frac{50 - 41}{4.92} = \frac{9}{4.92} \approx 1.83

  5. Find the Probability: Using the standard normal distribution, we find the probability corresponding to a Z-score of 1.83. The cumulative probability for Z=1.83Z = 1.83 is approximately 0.9664.

    So, the probability of X>50X > 50 is: P(X>50)=1P(Z1.83)=10.9664=0.0336P(X > 50) = 1 - P(Z \leq 1.83) = 1 - 0.9664 = 0.0336

Final Answer:

The probability that a majority of the 100 people say "yes" is approximately 0.0336, or 3.36%.


Would you like more details on any of the steps?

Here are a few follow-up questions:

  1. How does the normal approximation compare to the exact binomial distribution for this problem?
  2. What would happen if the probability of "yes" was closer to 0.5?
  3. How can we use the continuity correction in the normal approximation?
  4. What are other scenarios where the binomial distribution is approximated by a normal distribution?
  5. How does increasing the sample size affect the probability of a majority?

Tip: In problems with large nn, always consider using the normal approximation to simplify calculations, especially when the binomial distribution becomes tedious to compute directly.

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Math Problem Analysis

Mathematical Concepts

Probability
Binomial Distribution
Normal Approximation

Formulas

Binomial distribution: P(X = k) = C(n, k) * p^k * (1 - p)^(n - k)
Normal approximation to binomial: X ~ N(μ = np, σ^2 = np(1 - p))
Z-score: Z = (X - μ) / σ

Theorems

Normal Approximation to the Binomial Distribution
Central Limit Theorem

Suitable Grade Level

Grades 11-12 (Advanced Probability)