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1 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman M ARIO F. T RIOLA E IGHTH E DITION E LEMENTARY S TATISTICS Section 2-5 Measures of Variation

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2 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Waiting Times of Bank Customers at Different Banks in minutes Jefferson Valley Bank Bank of Providence

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3 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Jefferson Valley Bank Bank of Providence Jefferson Valley Bank Bank of Providence Mean Median Mode Midrange Waiting Times of Bank Customers at Different Banks in minutes

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4 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Figure 2-14 Dotplots of Waiting Times

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5 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Measures of Variation Range value highest lowest value

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6 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman a measure of variation of the scores about the mean (average deviation from the mean) Measures of Variation Standard Deviation

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7 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Sample Standard Deviation Formula Formula 2-4 ( x - x ) 2 n - 1 S =S =

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8 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Population Standard Deviation calculators can compute the population standard deviation of data 2 ( x - µ ) N =

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9 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Symbols for Standard Deviation Sample Population x x n s S x x n-1 Book Some graphics calculators Some non-graphics calculators Textbook Some graphics calculators Some non-graphics calculators Articles in professional journals and reports often use SD for standard deviation and VAR for variance.

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10 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Measures of Variation Variance standard deviation squared s 2 2 } Notation Sample Variance Population Variance

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11 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Sample Variance Population Variance Variance Formulas ( x - x ) 2 n - 1 s 2 = (x - µ)2 (x - µ)2 N 2 =

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12 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Round-off Rule for measures of variation Carry one more decimal place than is present in the original set of values. Round only the final answer, never in the middle of a calculation.

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13 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Estimation of Standard Deviation Range Rule of Thumb x - 2 s x x + 2 s Range 4 s or (minimum usual value) (maximum usual value)

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14 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Estimation of Standard Deviation Range Rule of Thumb x - 2 s x x + 2 s Range 4 s or (minimum usual value) (maximum usual value) Range 4 s

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15 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Estimation of Standard Deviation Range Rule of Thumb x - 2 s x x + 2 s Range 4 s or (minimum usual value) (maximum usual value) Range 4 s = highest value - lowest value 4

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16 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman minimum ‘usual’ value (mean) - 2 (standard deviation) minimum x - 2(s) maximum ‘usual’ value (mean) + 2 (standard deviation) maximum x + 2(s) Usual Sample Values

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17 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman x The Empirical Rule (applies to bell-shaped distributions ) FIGURE 2-15

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18 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman x - s x x + sx + s 68% within 1 standard deviation 34% The Empirical Rule (applies to bell-shaped distributions ) FIGURE 2-15

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19 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman x - 2s x - s x x + 2s x + sx + s 68% within 1 standard deviation 34% 95% within 2 standard deviations The Empirical Rule (applies to bell-shaped distributions ) 13.5% FIGURE 2-15

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20 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman x - 3s x - 2s x - s x x + 2s x + 3s x + sx + s 68% within 1 standard deviation 34% 95% within 2 standard deviations 99.7% of data are within 3 standard deviations of the mean The Empirical Rule (applies to bell-shaped distributions ) 0.1% 2.4% 13.5% FIGURE 2-15

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21 Chapter 2. Section 2-5. Triola, Elementary Statistics, Eighth Edition. Copyright Addison Wesley Longman Measures of Variation Summary For typical data sets, it is unusual for a score to differ from the mean by more than 2 or 3 standard deviations.

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