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1 Examining Relationships in Data William P. Wattles, Ph.D. Francis Marion University

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12 Examining relationships u Correlational design- –observation only –look for relationship –does not imply cause u Experimental design- A study where the experimenter actively changes or manipulates one variable and looks for changes in another.

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Dependent Variable u What we are trying to predict. u It measures the outcome of a study.

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Independent Variable u Is used to explain changes in the dependent variable

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15 Correlation u The relationship between two variables X and Y. u In general, are changes in X associated with Changes in Y? u If so we say that X and Y covary. u We can observe correlation by looking at a scatter plot.

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17 Scatterplot u Relationship between two quantitative variables u Measured on the same individual u Y axis is vertical u X axis horizontal u Each point represents the two scores of one individual

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18 Type of correlation u Positive correlation. The two change in a similar direction. Individuals below average on X tend to be below average on Y and vice versa. u Negative correlation the two change in the opposite direction. Individuals who are above average on X tend to be below average on Y and vice versa.

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Example of correlation from New York Times u Early studies have consistently shown that an "inverse association" exists between coffee consumption and risk for type 2 diabetes, Liu said. That is, the greater the consumption of coffee, the lesser the risk of diabetes

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19 Examples u Positive correlations: Hours spent studying and g.p.a.; height and weight, exam 1 score and exam 2 score, u Negative correlations; temperature and heating bills; hours spent watching TV and g.p.a.; SAT median and % taking the test. Age and price of used cars.

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20 Correlation Coefficient u One number that tells us about the strength and direction of the relationship between X and Y. u Has a value from -1.0 (perfect negative correlation) to +1.0 (perfect positive correlation) u Perfect correlations do not occur in nature

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21 Correlation Coefficient u The Pearson Product Moment Correlation Coefficient or Pearson Correlation Coefficient is symbolized by r. u When you see r think relationship.

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22 Strength of Correlation u Weak.10,.20,.30 u Moderate.40,.50,.60 u Strong.70,.80,.90 u No correlation 0.0

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23 Calculating a correlation coefficient u Deviation score for X (X-Xbar) u Deviation score for Y (Y-Ybar) u Standard deviations (SD) for X and Y u Number of subjects (n)

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24 Correlation Coefficient

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25 Correlation Coefficient

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26 Pearson Correlation Coefficient u Sum of (X-Xbar) times (Y-Ybar)/SD of X * SD of Y * n-1 u A Pearson correlation coefficient does not measure non-linear relationships u We represent the Pearson correlation coefficient with r.

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StudentNumber of Beers Blood Alcohol Level 150.1 220.03 390.19 670.095 730.07 930.02 1140.07 1350.085 480.12 530.04 850.06 1050.05 1260.1 1470.09 1510.01 1640.05 Here we have two quantitative variables for each of 16 students. 1. How many beers they drank, and 2. Their blood alcohol level (BAC) We are interested in the relationship between the two variables: How is one affected by changes in the other one?

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StudentBeersBAC 150.1 220.03 390.19 670.095 730.07 930.02 1140.07 1350.085 480.12 530.04 850.06 1050.05 1260.1 1470.09 1510.01 1640.05 In a scatterplot one axis is used to represent each of the variables, and the data are plotted as points on the graph.

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Explanatory (independent) variable: number of beers Response (dependent) variable: blood alcohol content x y Explanatory and response variables A response variable measures or records an outcome of a study. An explanatory variable explains changes in the response variable.

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Explanatory (independent) variable: number of beers Response (dependent) variable: blood alcohol content x y Explanatory and response variables Typically, the explanatory or independent variable is plotted on the x axis and the response or dependent variable is plotted on the y axis.

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27 Linear relationships u The Pearson correlation coefficient only works for linear relationships. u The assumption of linearity can be verified by examining a scatterplot. u Assumes that the relationship between X and Y is the same at different levels of X and Y

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Is mileage related to speed?

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Correlation? u Height u Inseam

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Obesity and soft drink consumption u APS Observer 10/2009 u As diet soft drink increases so does obesity u As regular soda increases so does obesity

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28 Correlation does not imply causation!

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Correlation =.59

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The End

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3 What is a Z score??

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4 What is the standard deviation??

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5 What percentage of observations lie within one standard deviation of the mean??

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6 What is the mean??

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7 What is the formula for the standard deviation??

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8 What percentage score less than a z-score of +1??

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9 What is a Z-score??

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10 What is the formula for a Z- score??

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17/21=.81

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4/21=.19 x 100= 19%

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One number that tells about the variability in the sample or population.

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How many standard deviations an individual's score lies above or below the mean.

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68%

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One number that tells us about the middle of the data, using all the data.

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84%

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How many standard deviations a score lies above or below the mean.

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The End

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