Length of a human pregnancy is 9 months. Mean is 266 days and standard deviation 16 days. What percentage of pregnancies last less than 250 days?

Question

Length of a human pregnancy is 9 months. Mean is 266 days and standard deviation 16 days. What percentage of pregnancies last less than 250 days?

Answer 1

We know that 250 is exactly one standard deviation below the mean because 266-16=250 (mean-sd).  Based on the empirical rule, 68% of the data in a normal curve exists within one standard deviation of the mean on either side.  Since 250 is one below, it is half of that area and therefore half of that 68%.  Since 50% lies below the mean of the data and we know that we are not including the 34% that is above 250, then you would subtract 50-34=16%.

 

ANSWER 16%