The length of human pregnancies from conception to birth can be approximated a normal distribution with a mean of 265 days and a standard deviation of 15 days. What percentage of all pregnancies will last between 250 and 280 days?
Correct Answer: 68%
Description: Normal/Gaussian curve:
Is ‘bilaterally symmetrical, bell-shaped’
Is bases on mean (µ) and standard deviation (σ)
Mean, Median and Mode coincide (Mean = Median = Mode)
Has Mean (µ) = 0 SD (σ) = 1
Mean ± 1SD (µ ± 1σ) covers 68% values
Mean ± 2SD (µ ± 2σ) covers 95% values
Mean ± 3SD (µ ± 3σ) covers 99% values
- In a Normal distribution, 50% of values lie above the mean and 50% lie below the mean.
Mean is 265 days , SD = 15
Mean + 1 SD = 250, 280
As Mean + 1 SD covers 68% values.
So percentage of all pregnancies will last between 250 and 280 days is 68%.
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