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Scatterplots & Correlation Section 3.1A
Scatterplots & Correlation
Section 3.1A
Relationships between two Variables
O A study found that short women are more likely to have heart attacks than tall women….
O Smokers on average die younger than nonsmokers….
O But – to make these conclusions we must first eliminate the effect of other variables.
Lurking VariablesO Can strongly influence the
relationship between two variables.
Case of the Missing Cookies
ScatterplotOUsed to show the relationship
between two quantitative variables measured on the same individuals. Each individual in the data appears as a point in the graph.O Explanatory variable goes on the
horizontal axis.O If there is no explanatory variable
then either variable can go on the horizontal axis.
Has the increase been constant?
Would Vote for a woman
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Years (since 1900)
% R
esp
on
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g Y
es
Series1
Types of Correlation
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0 10 20 30 40 500
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Describe the correlation
O Apples: circumference, weight
O College freshmen: shoe size, weight
O People: age, grip strength
O Drivers: blood alcohol, reaction time
Caution…..
Association does not imply causation!
Interpreting ScatterplotsO Look for direction (positive, negative, none)
O Look at the form of the relationshipO Straight or curvedO Any clusters
O Look at the StrengthO How closely does it follow the form
O Look for outliersO Individual value hat falls outside the overall
pattern of the relationship
When writing to describe:
O There appears to be a (strong, weak, moderate) (positive/negative) (linear, nonlinear) relationship between _____ (give the x variable) and ______ (give the y variable)
O Do not just say between x & y!
Graph Using a calculator:
Sprint Time (sec) Long Jump (in)5.41 1.715.05 1849.49 488.09 1517.01 907.17 656.83 946.73 788.01 715.68 1305.78 1736.31 1436.04 141
Interpret….
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Sprint (seconds)5 6 7 8 9
Period 5 Only Scatter Plot
1.Direction
2.Form
3.Strength
4.Outliers
Influential Pt!
The following data represents 9th grade students who go on a
backpacking trip.Body wt (lb) 120 187 109 103 131 165 158 116Backpack (lb) 26 30 26 24 29 35 31 28
Interpret: Backpack
80 100 120 140 160 180 2000
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Backpack (lb)
HomeworkOPage 159 (1-13) odd
