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5-Minute Check on Activity 7-10

- State the Empirical Rule:
- What is the shape of a normal distribution?
- Compute a z-score for x = 14, if μ = 10 and σ = 2
- What does a z-score represent?
- Which will have a taller distribution: one with σ = 2or σ = 4

Also known as 68-95-99.7 rule (± nσ’s from μ)

Symmetric mound-like

Z = (14-10)/2 = 2

Number of standard deviations away from the mean

Larger spread is smaller height; so σ = 2 is taller

Click the mouse button or press the Space Bar to display the answers.

Objectives

- Determine the area under the standard normal curve using the z-table
- Standardize a normal curve
- Determine the area under the standard normal curve using a calculator

Vocabulary

- Cumulative Probability Density Function – the sum of the area under a density curve from the left

Activity

Many high school students have part-time jobs after school and on weekends. Suppose the number of hours students spend working per week is approximately normally distributed, with a mean of 16 hours and a standard deviation of 4 hours. If a student is randomly selected, what is the probability that the student works between 12 and 18 hours per week?

Mean = 16 Standard Deviation (StDev) = 4

so one StDev below = 12 and ½ StDev above = 18

can use z-tables: P(12 < x < 18) = P( -1 < z < 0.5)

but using calculator is much easier!:

P(12 < x < 18) = normcdf(12, 18, 16, 4) = 0.5328

Normal Probability Density Function

There is a y = f(x) style function that describes the normal curve:where μ is the mean and σ is the standard deviation of the random variable x

In our example this gives us:

1

y = -------- e

√2π

-(x – μ)2

2σ2

1

y = -------- e

4√2π

-(x – 16)2

2∙42

Probability and Normal Curve

- All possible probabilities sum to 1
- Normal curve is a probability density function
- Area under the curve will sum to 1
- The area between two values is the probability that a value will occur between those two values
- Standard Normal is a normal curve with a mean of 0 and a standard deviation of 1
- Normal notation: X ~ N(μ,)

18

Activity contMany high school students have part-time jobs after school and on weekends. Suppose the number of hours students spend working per week is approximately normally distributed, with a mean of 16 hours and a standard deviation of 4 hours. If a student is randomly selected, what is the probability that the student works between 12 and 18 hours per week?

We want so we convert 12 and 18 to z-values

z12 = (12-16)/4 = -1 and z18 = (18-16)/4 = 0.5

Using Appendix C: P(z0.5)= 0.6915 and p(z-1)=0.1587

So P(12 < x < 18) = 0.6915 – 0.1587 = .5328 or 53.28%

Using Your TI-calculator

- Press 2ndVARS (DISTR menu)
- Press 2 (normalcdf)
- Parameters Required:
- Left value
- Right value
- Mean, μ
- Standard Deviation,
- Using your calculator, normcdf(left, right, μ, σ)
- Notes:
- Use –E99 for negative infinity
- Use E99 for positive infinity
- Don’t have to plug in 0,1 for μ, (it assumes standard normal)

Finding Area under any Normal Curve

- Draw a normal curve and shade the desired area
- Use your calculator, normcdf(left, right, μ, σ)

OR

- Convert the x-values to Z-scores using Z = (x – μ) / σ
- Draw a standard normal curve and shade the area desired
- Find the area under the standard normal curve using the table. This area is equal to the area under the normal curve drawn in Step 1

Summary and Homework

- Summary
- Normal Curve Properties
- Area under a normal curve sums to 1
- Area between two points under the normal curve represents the probability of x being between those two points
- Standard Normal Curves
- Appendix C has z-tables for cumulative areas
- Calculator can find the area quicker and easier
- TI-83 Help for Normalcdf(LB,UB,,)
- LB is lower bound; UB is upper bound
- is the mean and is the standard deviation
- Homework
- pg 881-883; problems 1, 3-5

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