Transcript Political Research and Statistics

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...
Select simple
Choose Define
A Window pops up
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