correlations 11/7/2013. readings chapter 8 correlation and linear regression (pollock) (pp. 182-187)...
TRANSCRIPT
Correlations
11/7/2013
Readings
• Chapter 8 Correlation and Linear Regression (Pollock) (pp. 182-187)
• Chapter 8 Correlation and Regression (Pollock Workbook)
Homework Due Today
• Chapter 7 Pollock Workbook – Question 1 • A, B, C, D, E, F
– Question 2 • A, B, C, D
– Question 3 (use the dataset from the homework page)• A, B, C, D
– Question 5 • A, B, C D, E
OPPORTUNITIES TO DISCUSS COURSE CONTENT
Office Hours For the Week
• When– Friday 10-12– Monday 10-12– Tuesday 8-12– And by appointment
Course Learning Objectives
1. Students will be able to interpret and explain empirical data.
2. Students will achieve competency in conducting statistical data analysis using the SPSS software program.
Correlations
What is correlation?
• Any relationship between two variables
• Correlation does not mean causation
What Could Be Happening?
• Variable A influences variable B
• Variable B influences variable A
• It is a coincidence
• Some other variable (C) influences both A and B
Measuring Pearson’s r
• Measure from -1 to 0 to 1. – -1 means a perfect
negative relationship – 0 is the absence of any
relationship – +1 is a perfect positive
relationship
• Like Somers’ D, Pearson's "r" scores tell us – Direction – Strength of Association – Statistical significance of
the measure
PEARSON'S r's are PRE Measures!
• Squaring the (r) value provides a measure of how much better we can do in predicting the value of the d.v by knowing the independent variable.
• We call this a r2 (r-square) value.
SCATTERPLOTS
A Way of Visualizing a Correlation
More on Scatterplots• We can think of this line as a
prediction line.
• The closer the dots to the line, the stronger the relationship, the further the dots the weaker the line.
• If all the data points are right on the regression line, then there is a perfect linear relationship between the two variables.
• This only graphs a correlation...... this means that it does not mean causality nor should it be used for testing!
CO2 and Urban Population
SCATTERPLOTS IN SPSS
How to do it
• Graphs • Legacy Dialogs • Scatter/Dot...
A Window pops upSelect simple
Choose Define
Adding Case Labels
• put your variable in the Label Cases by area
• Click on Options, and this will open up a window – Click on display chart
with case labels and continue • Click OK
Including a fit Line with your Scatterplot
Do not use scatterplots for testing! There are better measures, especially if you have more than 1 iv. (your paper should not include any scatterplots)
Lets try an example
• Use the following data set
• D.V. Obama• I.V. Unemp
• Follow the directions from last class
What is Going on?• The Line of Best Fit-
• How much error is in our line.
• A predictor for future values • Eyeballing the data, a state with
4% unemployment should give Obama how much support?
DATA
Primary data
• Collection
• Advantages
• Disadvantages
Secondary Data
• Collection
• Advantages
• Disadvantages