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AP Statistics: Section 2.2 C

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Example 1: Determine if each of the following is likely to have a Normal distribution (N) or a non-normal distribution (nn). _____ gas mileage of 2006 Corvettes _____ prices of homes in Westlake _____ gross sales of business firms _____ weights of 9-oz bags of potato chips

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While experience can suggest whether or not a Normal distribution is plausible in a certain case, it is very risky (especially on the AP test) to assume that a distribution is Normal without actually inspecting the data. Chapters 10 -15 of our text deal with inference, and an important condition that will need to be met is that the data comes from a population that is approximately Normal.

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Assessing the Normality of a Population from Sample Data

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Method 1: Construct a histogram, stemplotor dotplot. See if the graph is approximately ___________ and ____________. Non-Normal features to look for include ________, pronounced _________, or ______ and ________. bell-shaped symmetrical outliers skewness gaps clusters

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Because the data comes from a sample and not the population.

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Method 2: Construct a Normal probability plot. Statistical packages, as well as TI calculators, can construct a Normal probability plot. Before we look at constructing a Normal probability plot, let’s look at some basic ideas behind constructing a Normal probability plot.

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1. Arrange the observed data values from _________ to _________. Record what percentile of the data each value represents. For example, the smallest observation in a set of 20 would be at the _____ percentile, the second smallest would be at the _____ percentile, etc. smallestlargest 5 th 10 th

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2. Determine the z-scores for each of the observations. For example, z = _______ is the 5% point of the standard Normal distribution and z = _______ is the 10% point.

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3. Plot each data point x against the corresponding z. If the data distribution is close to Normal, the plotted points will be __________________. Systematic deviations from a straight line indicate a non-Normal distribution. approximately linear

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In a right-skewed distribution, the largest observations fall distinctly _______ a line drawn through the main body of points. In a left-skewed distribution, the smallest observations fall distinctly ________ a line drawn through the main body of points. Outliers appear as points that are far away from the overall pattern. above below

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Construct a Normal probability plot on your calculator as follows: 1. Enter the data in L1. (STATS, EDIT). 2. Go to STAT PLOT (2 nd Y=). ENTER. Highlight ON. First draw a histogram to see that the data appears to be Normally distributed. Key ZOOM 9 to get an appropriate window. 3. Go back to STAT PLOT. Change “Type” to the 6 th graph choice. For “Data List”, put in L1 and for “Data Axis”, highlight Y. 4. Push GRAPH and key ZOOM 9 to get an appropriate window.

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