Class Notes Ch 6 Sec 1 thru 3 - Ms. Zaleski's Math …mszaleski.weebly.com/uploads/1/3/1/5/13152548/class...Class Notes Ch 6 Sec 1 thru 3.pptx Author Terry Zaleski Created Date 5/1/2016
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+
The Normal Distribution
Chapter 6
+Normal Distributions Chapter 6, Section 1
+Normal Distributions
! In Chapter 5, we drew histograms representing probability distributions for discrete random variables.
! Instead let’s use a continuous variables such as height or weight of an adult woman. If we construct a histogram with the data for our sample, we might find that it is normally distributed.
! A normal distribution is also called a bell curve.
! A normal distribution can be used to describe many different variables.
! We will learn about the properties of the normal distribution in this chapter.
+The Shape of the Normal Distribution
+The Definition of a Normal Distribution
! If a random variable has a probability distribution whose graph is continuous, bell-shaped, and symmetric, it is called a normal distribution.
! The graph of a normal distribution is a normal distribution curve.
! The specific shape and position of a normal distribution curve depends on two things: ! The mean, μ
! The standard deviation, σ
+The Shape of a Normal Distribution
! The center of the normal distribution is the mean, μ.
! The spread of the normal distribution is determined by the standard deviation, σ.
+A Normal Distribution Comparison
! Which curve has the greater mean?
! Which curve has the greater standard deviation?
+Normal Distribution Curves
! Consider the normal curves shown at the right.
! Which curve has the greatest mean?
! Which curve has the greatest standard deviation?
+Skewed Distributions are NOT Normal
+The Normal Distribution
! The mean, median and mode are equal and are located in the center of the distribution.
! The normal curve is bell-shaped.
! The normal curve is symmetric about the mean, μ.
! The normal curve is continuous – there are no gaps or holes.
! The normal curve approaches, but never touches, the x-axis as it extends farther and farther away from from the mean.
! The total area under the normal curve is 1.00 or 100%.
Characteristics
+The Normal Distribution
! Between μ-σ and μ+σ (in the center of the curve) the graph curves downward.
! The graph curves upward to the left of μ-σ and to the right of μ+σ.
! The point at which the curve changes from curving upward to curving downward are called inflection points.
! Inflection points always occur at μ-σ and μ+σ.
Characteristics
+Examples of Normal Curves
! Notice that the mean, μ, is always the center.
! Notice that the points of inflection always occur at μ-σ and μ+σ.
+Recall the Empirical Rule…
" The area under the normal curve totals 1.00 or 100% " Recall that approximately 95% of the data values fall within μ± 2σ.
" This means that the area under the curve between μ- 2σ and μ+ 2σ is 0.95 or 95%.
+The Standard Normal Distribution
! The standard normal distribution is a normal distribution with a mean of 0 and a standard deviation of 1.
! The way we convert a normal distribution to a standard normal distribution is by using a z-score.
+The Standard Normal Distribution
! Remember that a z-score or z-value (also called standard score) is the number of standard deviations the data value is from the mean.
! Recall from chapter 3, that we used the following formula for a z-score:
! We will now calculate the z-values for a population using the following formula:
! Once the data values (X values) are transformed, using this formula, they become z-values.
z-values or z-scores
z =X − Xs
z =X − µ
σ
+The Standard Normal Distribution
! The area under the standard normal curve equals 1.00 or 100%.
! The area in any particular “section” of the standard normal curve represents the probability that a data value falls in that section.
+The Standard Normal Distribution
! We can use Table E (p.788) to find the area to the left of any z-value between -3.49 and 3.49.
" The area is close to 0 for z-scores close to z = -3.49.
" The area increases as z-scores increase.
" The area at z = 0 is 0.5 or 50%.
" The area is close to 1 for z-scores close to z = 3.49.
+Example: Finding Area to the LEFT
" Step 1: Draw a standard normal curve and shade the area.
" Step 2: Look up the z-value in the table and use the area given.
+Example: Finding Area to the RIGHT
" Step 1: Draw a standard normal curve and shade the area.
" Step 2: Look up the z-value in the table and subtract the area from 1.
+Example: Finding Area BETWEEN any TWO z-values
" Step 1: Draw a standard normal curve and shade the area.
" Step 2: Look up the two z-values in the table and subtract the small from the large.
+Notation…
! Recall that the area under the standard normal curve for a particular interval is the probability that a data value falls in that interval.
! For example if we want the area under the curve that falls between the z-values of 0 and 2.32, we are referring to the probability that a particular data value will fall between these two z-values.
! We can write this probability as P(0 < z < 2.32)
+Examples: Finding the Area Under the Standard Normal Distribution
+Example: Finding a z-value that Corresponds with an Area
+Applications of the Normal Distribution Section 6-2
+The Standard Normal Distribution and Practical Applications
! We can convert any variable that in normally distributed into a standard normal distribution variable (z-value ) by using the following formula.
! Consider a standardized test that has a mean of 100 and a standard deviation of 15. When the scores are transformed to z-values, the two distributions coincide.
z =X − µ
σ
+Steps For Finding the Area Under Any Normal Curve
Step 1: Draw a normal curve and
shade the desired area.
Step 2: Convert the values of X
(data values) to z-values using the
formula.
Step 3: Find the corresponding
area, using Table E on page 788.
z =X − µ
σ
+Example of an Application
+Example of an Application
+Example of an Application
+Finding Data Values Given Specific Probabilities
! Formula for finding a z-value is
! Formula for find a data value (X) is
z =X − µ
σ
X = z ⋅ σ + µ
+Finding Data Values for Specific Probabilities
Step 3: Calculate the X value by using the formula X = zσ + μ
Step 2: Find the z-value from Table E that corresponds to the desired probability (area).
Step 1: Draw a normal curve and shade the desired area that represents the probability, proportion or percentile.
+Example of an Application
+Example of an Application
+Determining Normality
! We may want to check if a distribution that we are working with is a normal distribution so that we can use the methods learned in this chapter.
! One way to check for normality is to draw a histogram for the data and check its shape. ! If the distribution is not approximately bell-shaped, the data are not
normally distributed.
! If the data is skewed, then it is NOT normally distributed. There is also a test to check if the data is skewed…
! Look for outliers. Recall that outliers are outside the range of Q1 – 1.5(IQR) and Q3 + 1.5(IQR) where the IQR = Q3 – Q1. The existence of outliers can have a big effect on normality.
+The Pearson Coefficient
! The Pearson coefficient (PC) can be calculated as follows:
A Test for Skewness
PC =3 X − median( )
s
PC ≥ 1 Data significantly skewed right
PC ≤ -1 Data significantly skewed left
PC = 0 Data not skewed
+Example of Determining Normality
+Example of Determining Normality
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