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Answer:
a) Due to the higher z-score, Ronald performed better relative to his peers on the test.
b) Ronald needed a grade of at least 732.5, and Rubin of at least 33.58.
c) 95% of the population fall between graded of 4.868 and 31.132 on the ACT.
95% of the population fall between graded of 304 and 696 on the SAT.
Step-by-step explanation:
Normal probability distribution
When the distribution is normal, we use the z-score formula.
In a set with mean and standard deviation
, the zscore of a measure X is given by:
The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.
In this question:
(a) Relative to their peers who also took the tests, who performed better on his test? Explain.
We have to find whoever has the higher z-score.
Ronald:
Ronald scores 700 on the math section of the SAT exam. The distribution of SAT scores is approximately normal with a mean of 500 and a standard deviation of 100. So the z-score is found when . So
Rubin:
Rubin takes the ACT math exam and scores 31 on the math portion. ACT scores are approximately normally distributed with a mean of 18 and a standard deviation of 6.7. So the z-score is found when . So
Due to the higher z-score, Ronald performed better relative to his peers on the test.
(b) A certain school will only consider those students who score in the top 1% in the math section. What grades would Ronald and Rubin have to receive on their respective tests to be considered for admission?
They have to be in the 100 - 1 = 99th percentile, that is, they need a z-score with a pvalue of at least 0.99. So we need to find for them X when Z = 2.325.
Ronald:
Rubin:
Ronald needed a grade of at least 732.5, and Rubin of at least 33.58.
(c) Between what two grades does 95% of the population fall for the ACT and the SAT exams?
They fall between the 100 - (95/2) = 2.5th percentile and the 100 + (95/2) = 97.5th percentile, that is, they fall between X when Z = -1.96 and X when Z = 1.96.
ACT:
Lower bound:
Upper bound:
95% of the population fall between graded of 4.868 and 31.132 on the ACT.
SAT:
Lower bound:
Upper bound:
95% of the population fall between graded of 304 and 696 on the SAT.