Probability of normal distribution

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SUMMARY

The discussion focuses on finding the normal approximation for the binomial probability P(x = 4, 5) with parameters n = 14 and p = 0.5. The calculations yield a mean (μ) of 7 and a standard deviation (σ) of 1.87, leading to a z-score of -1.61. The calculated probability using the normal approximation is 0.0537, while the exact probability is 0.0148904, indicating a significant error in approximation. The normal approximation is deemed inadequate for smaller sample sizes, as demonstrated by the 35% error margin when N is not sufficiently large.

PREREQUISITES
  • Understanding of binomial distributions
  • Knowledge of normal distribution and its properties
  • Familiarity with z-scores and standard deviation calculations
  • Ability to apply continuity correction in normal approximations
NEXT STEPS
  • Study the Central Limit Theorem and its implications for normal approximations
  • Learn about continuity correction techniques in normal approximations
  • Explore the impact of sample size on the accuracy of normal approximations
  • Investigate the differences between exact binomial probabilities and normal approximations
USEFUL FOR

Students studying statistics, educators teaching probability theory, and data analysts working with binomial distributions and normal approximations.

g.lemaitre
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Homework Statement



find the normal approximation for the binomial probability P(x = 4,5) where n=14 and p = .5.

Homework Equations



μ = np
σ = sqrt(npq)

z = (x - μ)/σ

The Attempt at a Solution



p = .5 q = .5

μ = 14*.5 = 7

σ = sqrt(14 * .5 * .5) = 1.87

z = (4 - 7)/1.87 = -1.61

(my book uses tables to convert the z score into the probability of getting x < 4)

z = -1.61 = .5 - .4463 = .0537

The book says the answer is .1812 which is not what I'm getting.


z =
 
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it's rare that such a simple problem takes this long. let me provide the example from the book

Screenshot2012-10-06at63129PM.png


Screenshot2012-10-06at63132PM.png
 
g.lemaitre said:
it's rare that such a simple problem takes this long. let me provide the example from the book

Screenshot2012-10-06at63129PM.png


Screenshot2012-10-06at63132PM.png

The normal approximation is not very good in this example (because N = 25 is not very large and z is more than 2 standard deviations below the mean). P_exact = 0.0148904, while the continuity-corrected normal approximation is about 0.020152 (so using the normal gives about a 35% error). The normal approximation would be better if N were larger or z were closer to the mean.

RGV
 

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