handout_2_1_fall11 - with high values of the other...

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Chapter 2.1- Scatterplots •we often care about the relationship between two variables •to study the relationship, we measure both variables on the same individuals response variable - measures an outcome of a study ( y ) explanatory variable - explains or causes change in the response variable ( x ) dependent variables - response variables ( y ) independent variables - explanatory variables ( x ) scatterplot - shows the relationship between two quantitative variables measured on the same individuals •plot explanatory variable on horizontal axis ( x axis) •plot response variable on vertical axis ( y axis) Student Beers BAC 1 5 0.1 2 2 0.03 3 9 0.19 6 7 0.095 7 3 0.07 9 3 0.02 11 4 0.07 13 5 0.085 4 8 0.12 5 3 0.04 8 5 0.06 10 5 0.05 12 6 0.1 14 7 0.09 15 1 0.01 16 4 0.05 Scatterplots 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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Form and direction of an association Linear Nonlinear No relationship Positive association: High values of one variable tend to occur together
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Unformatted text preview: with high values of the other variable. Negative association: High values of one variable tend to occur together with low values of the other variable. 2 No relationship: X and Y vary independently. Knowing X tells you nothing about Y . Strength of the association The strength of the relationship between the two variables can be seen by how much variation, or scatter, there is around the main form. With a strong relationship, you can get a pretty good estimate of y if you know x. With a weak relationship, for any x you might get a wide range of y values. 3 Figure 2.1- Mean State SAT Score Versus Percent Taking SAT Describe overall pattern: 2 clusters: 1) high % take low scores (SAT states- broad group takes) 2) low % take high scores (ACT states- select group takes) Describe pattern within cluster: ACT states- roughly linear negative relationship SAT- weak linear relationship 4...
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handout_2_1_fall11 - with high values of the other...

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