Arraryeldergox2

2022-06-25

I was given a question where I was supposed to find the probability of obtaining y between two scores, however when I input my answer it tells me that I'm wrong, the question is given below along with my answer to the question:

Question

For a normal distribution with sample mean =−19 and standard deviation =6.85 find $p(-16.15\le y\le -15.27)$, where y is a random draw from the normal distribution. Round to 4 decimal places.

My answer

I obtained the z scores for both the y values −15.27 and −16.15 and their respective z scores are 0.5445 and 0.4161. The probability under the curve with a z score of 0.5445 is 0.7054 and the probability under the curve with a z score of 0.4161 is 0.6628. With those two probabilities, I obtained the difference (getting 0.0426) and I put in that answer and was told that I was wrong. Can someone give me insight as to what I'm doing wrong?

Question

For a normal distribution with sample mean =−19 and standard deviation =6.85 find $p(-16.15\le y\le -15.27)$, where y is a random draw from the normal distribution. Round to 4 decimal places.

My answer

I obtained the z scores for both the y values −15.27 and −16.15 and their respective z scores are 0.5445 and 0.4161. The probability under the curve with a z score of 0.5445 is 0.7054 and the probability under the curve with a z score of 0.4161 is 0.6628. With those two probabilities, I obtained the difference (getting 0.0426) and I put in that answer and was told that I was wrong. Can someone give me insight as to what I'm doing wrong?

Alisa Gilmore

Beginner2022-06-26Added 22 answers

The problem is your CDF table for the normal distribution only allows you to get a couple decimals, so you're saying $\mathrm{\Phi}(.5445)\approx \mathrm{\Phi}(.54)=.7054$ which (apparently) isn't a good enough approximation.

$\mathrm{\Phi}(.5445)-\mathrm{\Phi}(.4161)\approx .706951-.661332=.045619\approx .04562$

$\mathrm{\Phi}(.5445)-\mathrm{\Phi}(.4161)\approx .706951-.661332=.045619\approx .04562$

Bailee Short

Beginner2022-06-27Added 3 answers

You haven't maintained enough numerical precision. Specifically, it looks like you rounded your z-scores to two significant figures. Your result is the probability of a z-score between 0.42 and 0.54, not between 0.4161 and 0.5445.

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