Question

A researcher collected data from a sample of correctional officers and found that on average they made 47,173 dollars a year with S=6,364. (10 points) a. Calculate the z-score for a correctional officer who makes 40,000 dollars a year b. Calculate the z-score for a correctional officer who makes 50,000 dollars a year? c. What proportion of correctional officers make more than 40,000 dollars a years?

Answer 1

Answer:

a) $z = \frac{40000- 47173}{6364} = -1.127$

b) $z = \frac{50000- 47173}{6364} = 0.444$

c) $P(X>40000)=P(\frac{X-\mu}{\sigma}>\frac{40000-\mu}{\sigma})=P(Z>\frac{40000-47173}{6364})=P(z>-1.127)$

And we can find this probability using the complement rule and the normal standard distribution or excel and we got:

$P(z>-1.127)=1-P(z$

Step-by-step explanation:

Previous concepts

Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".

The Z-score is "a numerical measurement used in statistics of a value's relationship to the mean (average) of a group of values, measured in terms of standard deviations from the mean".  

Part a

Let X the random variable that represent the amount of money of a population of interet, and we know the following info:  

Where $\bar X=47173$ and $s=6364$

We can assume that the sample size is large and the estimators can be used as a good description for the parameters  $\mu \sigma$

The z score for 40000 would be:

$z = \frac{40000- 47173}{6364} = -1.127$

Part b

The z score for 50000 would be:

$z = \frac{50000- 47173}{6364} = 0.444$

Part c

We are interested on this probability

$P(X>40000)$

And the best way to solve this problem is using the normal standard distribution and the z score given by:

$z=\frac{x-\mu}{\sigma}$

If we apply this formula to our probability we got this:

$P(X>40000)=P(\frac{X-\mu}{\sigma}>\frac{40000-\mu}{\sigma})=P(Z>\frac{40000-47173}{6364})=P(z>-1.127)$

And we can find this probability using the complement rule and the normal standard distribution or excel and we got:

$P(z>-1.127)=1-P(z$