correlations 11/7/2013. readings chapter 8 correlation and linear regression (pollock) (pp. 182-187)...

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