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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Chapter 16 Descriptive Statistics

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Tell whether the following statement is true or false: Nominal measurement is the ranking of objects based on their relative standing on an attribute.

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer Nominal measurement: the classification of characteristics into mutually exclusive categories Ordinal measurement: the ranking of objects based on their relative standing on an attribute

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Measurement Nominal measurement: the classification of characteristics into mutually exclusive categories Ordinal measurement: the ranking of objects based on their relative standing on an attribute Interval measurement: indicating not only the ranking of objects but the amount of distance between them Ratio measurement: distinguished from interval measurement by having a rational zero point

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Descriptive Statistics Summarize quantitative data Describe quantitative data

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Tell whether the following statement is true or false: Frequency distributions impose order on raw data. Numeric values are ordered from lowest to highest, accompanied by a count of the number of times each value was obtained.

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer True Frequency distributions impose order on raw data. Numeric values are ordered from lowest to highest, accompanied by a count of the number (or percentage) of times each value was obtained.

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Frequency Distributions Impose order on raw data. Numeric values are ordered from lowest to highest, accompanied by a count of the number (or percentage) of times each value was obtained. Common methods of display –Histograms –Frequency polygons

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Distribution Symmetric: two halves are mirror images of each other Skewed: Asymmetric with one tail longer than the other –Positively skewed –Negative skewed Modality: number of peaks –Unimodal –Multimodal Normal distribution

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Question What is the median? A.Average or typical value of a set of scores B.Value that occurs most frequently in a distribution C.Point above which and below which 50% of the cases fall D.Arithmetic average of all scores

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer C Central tendency are indexes, expressed as a single number, that represent the average or typical value of a set of scores. The mode is the value that occurs most frequently in a distribution, the median is the point above which and below which 50% of the cases fall, and the mean is the arithmetic average of all scores.

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Central Tendency Central tendency are indexes, expressed as a single number, that represent the average or typical value of a set of scores. Mode: value that occurs most frequently in a distribution Median: point above which and below which 50% of the cases fall Mean: arithmetic average of all scores

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Tell whether the following statement is true or false: Range is distance between the highest and lowest scores

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer True Range is distance between the highest and lowest scores.

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Variability Variability: how spread out the data is Range: distance between the highest and lowest scores Standard deviation: indicates how much, on average, scores deviate from the mean Calculation: –Deviation scores represent the degree to which each person’s score deviates from the mean. The variance is equal to the SD squared.

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Bivariate Descriptive Statistics Relationships between two variables Contingency table two-dimensional frequency distribution in which the frequencies of two nominal- or ordinal-level variables are crosstabulated. Correlation coefficients describe the direction and magnitude of a relationship between two variables. –Product moment correlation coefficient (Pearson’s R)—interval or ratio –Spearman rho coefficient—ordinal Scatter plot

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Risk Indexes Absolute risk reduction: expresses the estimated proportion of people who would be spared from an adverse outcome through exposure to an intervention Relative risk: expresses the estimated proportion of people who would be spared from an adverse outcome through exposure to an intervention Relative risk reduction: estimated proportion of untreated risk that is reduced through exposure to the intervention

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Copyright © 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins Risk Indexes Odds ratio: ratio of the odds for the treated versus untreated group Number needed to treat: estimate of how many people would need to receive the intervention to prevent one adverse outcome

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