Question

Data are drawn from a bell-shaped distribution with a mean of 25 and a standard deviation of 4. There are 1,000 observations in the data set.
a. Approximately what percentage of the observations are less than 33?
b. Approximately how many observations are less than 33?

Answer 1

Answer:

a. 97.72% of the observations are less than 33

b. Approximately 977 observations are less than 33.

Step-by-step explanation:

Normal Probability Distribution:

Problems of normal distributions can be solved using the z-score formula.

In a set with mean $\mu$ and standard deviation $\sigma$, the z-score of a measure X is given by:

$Z = \frac{X - \mu}{\sigma}$

The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the p-value, we get the probability that the value of the measure is greater than X.

Mean of 25 and a standard deviation of 4.

This means that $\mu = 25, \sigma = 4$

a. Approximately what percentage of the observations are less than 33?

The proportion is the p-value of Z when X = 33. So

$Z = \frac{X - \mu}{\sigma}$

$Z = \frac{33 - 25}{4}$

$Z = 2$

$Z = 2$ has a p-value of 0.9772

0.9772*100% = 97.72%

97.72% of the observations are less than 33.

b. Approximately how many observations are less than 33?

Out of 1000:

0.9772*1000 = 977.2

Approximately 977 observations are less than 33.