9 - correlational research designs

9 correlational - Associations Among Quantitative Variables 02:47 Scatterplot Uses a standard coordinate system in which the horizontal axis

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Associations Among Quantitative Variables 02:47 Scatterplot: Uses a standard coordinate system in which the horizontal axis  indicates the scores on the predictor variable and the vertical axis represents the  scores on the outcome variable  A point is plotted for each individual at the intersection of his or her scores on  the two variables Regression line: Line of “best fit” The line that minimizes the squared distance of the points from the line  Linear Relationships When the variables on the scatterplot can be easily approximated with a  straight line  o Positive linear: When the straight line indicates that individuals who have  above-average values on one variable tend to have above-average values  on the other variable  o Negative linear: Occur when above-average values on one variable tend to  be associated with below-average values on the other variable Nonlinear Relationships Relationships between variables that are not well described with a straight line  o Independent: No relationship at all between the two variables We cannot use one variable to predict the other Curvilinear relationships: Relationships that change in direction and thus are  not described by a single straight line
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Statistical Assessment of Relationships 02:47 The Pearson Correlation Coefficient Normally used to summarize and communicate the strength and direction of  the association between two quantitative variables o Number that indicates both the direction and the magnitude of the  association  The direction of the relationship is indicated by the sign of the correlation  coefficient  o Positive values of r indicate that the relationship is positive linear o Negative values of r indicate negative linear relationships  o The strength or effect size of the linear relationship is indexed by the  distance of the correlation coefficient from zero  Interpretation of r o A significant r indicates that there is a linear association between the 
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This note was uploaded on 11/16/2010 for the course PSYC 420 taught by Professor Staff during the Fall '08 term at Maryland.

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9 correlational - Associations Among Quantitative Variables 02:47 Scatterplot Uses a standard coordinate system in which the horizontal axis

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