It is estimated that the probability that, at least 77 men are correctly identified by their marital status is approximately 0.0166.
How does the Normal Approximation to the Binomial work?For a random variable (in this case, the identification of men by their marital status) resulting in a "success" (i.e., correctly identified) or "failure" (i.e., incorrectly identified) for finite trials, which is claimed to follow a binomial distribution, is approximated to the normal probability distribution, i.e. X ∼ Binomial (c,p) → X ∼ Normal (μ, σ²), if np ≥ 5 or n (1-p)≥5.
Where:
n = Number of Trials; and
p = probability of success.
From the information given we have 100 men, that is:
n = 100,
P = 67% = 0.67
Thus,
np = 100 * 0.67 = 67 (> 5)
And n(1-p) = 100 * (1-0.67) = 33(>5), the normal approximation is possible.
Thus,
μ = np = 100 * 0.67 = 67
σ = √(np(1-p))
= √[100*0.67*(1-0.67)]
[tex]\approx[/tex] 4.7021
Let's say X = number of men identified by their marital status correctly.
For X = 77,
Z = [X-μ]/σ
= (77-67)/4.7021
[tex]\approx[/tex] 2.13
We estimate the probability that, at least 77 men are correctly identified by their marital status, i.e.,
Probability = (X ≥ 77)
P (Z≥2.13)
= P (Z≥0) - P(0≤Z<2.13)
= 0.5 - 0.483414193316395 (area table of the normal curve)
[tex]\approx[/tex] 0.0166
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