Question

A study reported that the length of pregnancy from conception to birth is approximately normally distributed with mean of 272 days and standard deviation of 9 days. What proportion (or %) of pregnancies last longer than 280 days?

Answer 1

Given Mean = 272 days

Standard Deviation = 9 days

We need to the find the proportion of pregnancies last longer than 280 days which is P(X>280)

Z-score = (X - ) /

z-score = (280 - 272) / 9

= 8/9

= 0.888889

P(X>280) will be area to the right of z-score 0.888889. The z-tables will have areas only for z-scores rounded to 2 decimal places. So here we will use online calculator to get the exact area to the right of z-score

The area to the right of z-score 0.888889 = 0.1870314

So proportion of pregnancies last longer than 280 days = 0.1870314 or 18.70314%