Question

According to the WHO report, girls who are 2-years old have a mean head circumference of 47.18cm with a standard deviation of 1.40 cm. A two year old girl with Down's syndrome has a head circumference of 44.5 cm. Children with Down's syndrome typically have smaller heads, so this is not surprising. 1. Relative to the WHO data, what is this girls z-score? What does the z score tell us? 2. Using the WHO data in a normal model, what percentage of the girls has a head circumference that is smaller than the girl with Down's Syndrome?

Answer 1

1. z = -1.91429

The z score tells us that the head circumference of the girl with the down syndrome ( 44.5 cm) is 1.91429 standard deviations below the mean or average head circumference

2. 2.7792%

Explanation:

1. Relative to the WHO data, what is this girls z-score?

z score formula is:

z = (x-μ)/σ, where

x is the raw score = 44.5 cm

μ is the population mean = 47.18cm

σ is the population standard deviation = 1.40cm

z = 44.5 - 47.18/1.40

z = -1.91429

What does the z score tell us?

The z score tells us that the head circumference of the girl with the down syndrome ( 44.5 cm) is 1.91429 standard deviations below the mean or average head circumference

2. Using the WHO data in a normal model, what percentage of the girls has a head circumference that is smaller than the girl with Down's Syndrome?

z score = -1.91429

Probability value from Z-Table:

P(z =-1.91429) = P(x<44.5) = 0.027792

Converting to percentage = 0.027792 × 100

= 2.7792%