Transcript Linear Regression - The Joy of Stats

Linear Regression (Bivariate)
Creating a Model to Predict an Outcome [1]
If we wanted to predict the win/loss record for each
team in a league, what variables would we
consider?
 This would require a very complex model —
there are many variables involved.
 For starters, we will look at a two-variable
model with one predictor variable and one
outcome variable, for example, using the past
performance of the quarterbacks to predict the
teams’ performances.
Creating a Model [2]
 How would you operationalize quarterback
performance as a single interval-ratio predictor
variable?
 How would you operationalize a team’s win/loss
record as an interval-ratio variable?
What is the Question?
 Are these two variables related?
 Does knowing about the distribution of the
predictor variable (or IV) allow us to estimate
values of the outcome variable (or DV)?
 Can we write the equation of a line to represent
the relationship?
 Are these estimates any better than just
guessing the mean of the DV distribution?
What Type of Variables?
Both variables should be at the interval-ratio level
of measurement.
Examples?
Examples: Two-Variable Questions
 Can we estimate individuals’ weights if we know
their heights?
 Can we estimate how long it takes a person to
get to class (time) if we know how far people
live (distance)?
 If we know students’ high school GPAs, can we
estimate (or predict) their college GPAs?
Examples: Places or Organizations
as the UNITS OF ANALYSIS (cases)
 Can we estimate countries’ infant mortality rates
if we know the number of physicians per 1,000
people?
 Are female literacy rates related to male life
expectancies, for countries?
 Are cities’ unemployment rates related to their
homicide rates?
 Are the number of books in the libraries of
various colleges a good predictor of the
incomes of the alumni from each of those
colleges?
First Step: Univariate Analysis
 Look at the distribution of each variable
separately.
 Use histograms, boxplots, descriptives, and
other SPSS/PASW functions (e.g., Analyze–
Statistics–Explore).
 A very skewed or otherwise “non-normal”
variable may not be suitable for the linear
regression.
Second Step: The Scatterplot
 IV (predictor variable) on the x-axis.
 DV (outcome variable) on the y-axis.
 Each point is a case located by its X score and
its Y score (ordered pair).
Does the shape of points look “sort of” linear?
 Put the line into the chart.
What does it mean for a point (case) to be above
or below the line?
Direction of the Relationship:
Positive and Negative Slopes
A positive relation will show up as a line with
positive slope, from lower left to upper right.
A negative or inverse relationship will show up as a
line with negative slope, from upper left to lower
right.
Whether a relationship is positive or negative will
be apparent from the sign of R, the correlation
coefficient.
Pearson’s R:
The Correlation Coefficient [1]
If the plot looks “sort of linear”
Find R (Pearson’s correlation coefficient), which
ranges from –1 to +1.
 0 means no relationship.
 –1 means a perfect negative or inverse
relationship.
 +1 means a perfect positive relationship.
Correlation Coefficient [2]
The correlation coefficient is NOT a percentage or
proportion.
R = ∑ [ZxZy] / N
R expresses the strength and the direction of the
relationship.
The Direction of the Relationship: Positive
The maximum, +1, is reached, if for every case,
Zx = Zy
In a positive relationship, Z-scores will be
multiplied together for a positive product, and
negative Z-scores will be multiplied together for
a positive product.
An example is heights and weights.
The Direction of the Relationship:
Negative
The minimum, –1, will be obtained when, for each
case,
Zx = –Zy
In a negative relationship, each product will involve
multiplying two Z-scores with opposite signs
(negative and positive), and the product will be
negative.
An example from the country data set is literacy
rate and infant mortality rate.
Interpreting R
Not all texts agree on how to interpret the strength
of R.
See Garner (2010, p. 173) for a commonly used
interpretation.
R2: The Coefficient of Determination
Square R to obtain R2 which is called the
coefficient of determination.
 R2 ranges from 0 to 1, and it can be read as a
proportion. (Or move the decimal point two
places to the right, and read it as a percent.)
 It reveals what proportion of the variation in
the outcome variable was predicted by the
predictor variable.
R2 is a Proportion between Variances
Three variances:
 Total variance (difference between mean and Y).
 Explained regression variance (difference
between mean and estimated Y).
 Unexplained residual or error variance (difference
between estimated and observed Y).
R2 is the ratio of explained, regression variance to
total variance.
See Figure 19 in Garner (2010), p. 177.
Why is R2 So Important?
 If R2 is 0, it means that the predictor variable is
worthless as a predictor. Our best estimate of
the outcome (DV) variable remains the mean of
the DV. This situation would look like a flat line
at the mean of the Y-distribution for the total fit
line in the scatterplot.
 A large R2 means that the linear model provides
good estimates of the dependent variable —
better than guessing the mean.
Why is There an ANOVA in My
Regression Analysis?
 The ANOVA “box” we see in the middle of the
regression output is a test (F-test) for the
significance of R2.
 It is testing the null hypothesis that the predictor
variable does NOT predict any of the variation
we found in the outcome variable’s distribution.
R2 is a Proportion
R2 = regression variance / total variance
It answers these questions:
 How good is my model (the regression line) for
predicting the distribution of the dependent
(outcome) variable?
 How close are the observed Ys to the Y′s
estimated in the linear model?
The Regression Coefficients
Our linear model of the relationship of the two
variables is written as the equation of a line.
Y = a + bX
 The y-intercept (constant) is a.
 The slope of the line is b.
 We are mostly interested in b.
The Table of Coefficients
In SPSS/PASW output, the table of coefficients
shows the constant term and the slope coefficient.
(They are under the heading “B”.)
It shows the standardized and unstandardized
slope coefficients.
 The standardized coefficients are called the
betas or beta-weights, and are based on the Zscores for each of the two distributions.
 Each coefficient is tested for significance with a
t-test. The null hypothesis is that beta = 0.
The Regression Model [1]
In the real world, the constant term is often
meaningless, and we are interested in it only for
writing the equation.
We want to know if b is significant.
 If it is not, forget about it — the whole analysis
is off (and R and R2 will also be NOT
significant).
 If it is, we can write the equation with either a
standardized or an unstandardized coefficient.
The Regression Model [2]
The unstandardized coefficients express the
relationship in terms of “real world” units of
measurement — e.g., feet, kilos, metres, inches,
minutes, literacy percentage points, and books in
libraries.
The standardized coefficient expresses the
relationship in terms of the Z-scores of the two
variables.
Positive and Negative (Inverse)
Relationships [1]
If the slope coefficient is a positive number, it
expresses a positive relationship between the
variables — more of one is associated with more
of the other.
(For example, more study time, higher GPA)
If the slope coefficient is a negative number, it
expresses a negative or inverse relationship
between the variables — more of one is
associated with less of the other.
(For example, more binge drinking, lower GPA)
Positive and Negative [2]
Positive relationships will have a positive slope for
the line in the graph, a positive slope coefficient,
and a positive R.
Negative relationships will have a negative slope
for the line in the graph, a negative slope
coefficient, and a negative R. (Warning: This
negative R is missing its minus sign in some parts
of SPSS/PASW output.)
R2 is always 0 or positive.
The End
Now we have our linear model BASED ON THE
AVAILABLE DATA. If R, R2, and the slope
coefficients are significant, we have improved our
ability to predict the outcome. Our estimates, Y′,
are better than just guessing the mean of the Y
distribution. We can return to our first question and
start the analysis:
 Is the past performance of the quarterbacks a
good predictor of their teams’ records in the
coming season?