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greyxxamber
30.05.2020 •
Mathematics
The lengths of pregnancies are normally distributed with a mean of 250 days and a standard deviation of 15 days.
a. Find the probability of a pregnancy lasting 308 days or longer?
b. If we stipulate that a baby is premature if the length of the pregnancy is in the lowest 8% (8th percentile), find the length that separates premature babies from those who are not premature.
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Ответ:
a) 0.005% probability of a pregnancy lasting 308 days or longer
b) The pregnancy length that separates premature babies from those who are not premature is 229 days.
Step-by-step explanation:
Problems of normally distributed samples can be solved using 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, we have that:
a. Find the probability of a pregnancy lasting 308 days or longer?
This is 1 subtracted by the pvalue of Z when X = 308. So
1 - 0.99995 - 0.00005
0.005% probability of a pregnancy lasting 308 days or longer
b. If we stipulate that a baby is premature if the length of the pregnancy is in the lowest 8% (8th percentile), find the length that separates premature babies from those who are not premature.
The 8th percentile is X when Z has a pvalue of 0.08. So it is X when Z = -1.405.
The pregnancy length that separates premature babies from those who are not premature is 229 days.
Ответ: