psyc 331 statistics for psychologists · 2017. 9. 19. · college of education school of continuing...
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College of Education
School of Continuing and Distance Education 2014/2015 – 2016/2017
PSYC 331
STATISTICS FOR PSYCHOLOGISTS
Session 4– A PARAMETRIC STATISTICAL
TEST FOR MORE THAN TWO
POPULATIONS
Lecturer: Dr. Paul Narh Doku, Dept of Psychology, UG Contact Information: [email protected]
godsonug.wordpress.com/blog
Session Overview
This session builds upon previous sessions and
provides further insight into some parametric
statistical concepts that will help in the
testing of hypotheses.
The goal of this session is to equip students
with the ability to
explain the terminology of analysis of variance
(ANOVA) ;
compute Fobs; Why Fobs should equal 1 if H0 is
true, and why it is greater than 1 if H0 is false;
Dr. P. N. Doku, Slide 2
Session Outline
The key topics to be covered in the session are as follows:
• The analysis of variance (ANOVA) procedure
• The general logic of ANOVA
• Computational procedures
• Post-hoc analysis: Multiple comparisons following the ANOVA test
• Worked example and exercises based on the One-Way ANOVA test
• Introduction to Two-Way analysis of variance (Two-Way ANOVA)
Slide 3
Reading List
• Opoku, J. Y. (2007). Tutorials in Inferential Social Statistics. (2nd Ed.). Accra: Ghana Universities Press. Pages 85 - 109
Slide 4
Analysis of Variance
• The analysis of variance is the parametric
procedure for determining whether significant differences occur in an experiment with three or more sample means
• However, in a research study of experiment involving only two conditions of the independent variable (two samples means), you may use
either a t-test or the ANOVA and the outcome of the analysis will be the same.
Slide 5
Experiment-Wise Error
Slide 6
• The probability of making a Type I error over a series of individual statistical tests or comparisons in an experiment is called the
experiment-wise error rate
• When we use a t-test to compare only two means in an experiment, the experiment-
wise error rate equals
Experiment-Wise Error
Slide 7
• When there are more than two means in an
experiment, the multiple t-tests result in an
experiment-wise error rate much larger than
the we have selected
• Using the ANOVA allows us to make all our decisions and keep the experiment-wise error
rate equal to
An Overview of One Way ANOVA
• ANalysis Of VAriance is abbreviated as ANOVA
• ANOVA is also called the F ratio
• There is a single independent variable, hence
called One-Way
• An independent variable is also called a factor
• Each condition of the independent variable is called a level or treatment
• Differences produced by the independent variable are treatment effect
Slide 8
Requirements for using the F ratio
1) Must be a comparison between three or more
means.
2) Must be working with interval data.
3) Our sample must have been collected randomly from
the research population.
4) We can/must assume that the sample characteristics
are normally distributed.
5) We must assume that the variance between samples
are all equal. Slide 9
10
Between-Subjects
• A one-way ANOVA is performed when one
independent variable is tested in the experiment
• When an independent variable is studied using
independent samples in all conditions, it is called a
between-subjects factor
• A between-subjects factor involves using the formulas
for a between-subjects ANOVA
11
Within-Subjects Factor
• When a factor is studied using related (dependent) samples in all levels, it is called a within-subjects
factor
• This involves a set of formulas called a within-subjects
ANOVA
12
Diagram of a Study Having
Three Levels of One
Factor
13
Null and Alternate Hypotheses
• Null hypothesis
H0 : 1 2 ... k
• Alternate hypothesis: states that at least the means of two of the populations differ.
H a : not all k are equal
The ANOVA (F) Test
• The statistic for the ANOVA is F
• When Fobs is significant, it indicates only that somewhere among the means at least two of them differ significantly
• It does NOT indicate which specific means
differ significantly
• When the F-test is significant, we perform post hoc comparisons to determine which specific means differ significantly
Computation of the ANOVA (F) Test
The Analysis of Variance is a multi-step process.
1. Sum of Squares 2. Mean Square 3. F Ratio
Slide 15
Sum of Squares
• The computations for the ANOVA require the
use of several sums of squared deviations
• The sum of squares is simply the sum of the
squared deviations of a set of scores around
the mean of those scores
• Adding them up.
• It is symbolized as SS
Sum of Squares
Comparing Groups: When groups are compared, there are more than one type of sum of squares.
Total Sum of Squares (SS total)
Between Groups Sum of Squares (SS between)
Within Groups Sum of Squares (SS within)
Each type represents the sum of squared
deviations from a mean.
Computational Formulae for SS
Slide 18
X X 2 N SS T
2
X12 X 2 2 Xk 12 Xk 2 X 2
N nk nk 1 n2 n1
SSB ...
X12 X 2 2 Xk 12 Xk 2
k
k 1 Xk
... Xk 1
SSW X1
X 2
n n n n
2 2
2
2
1
2
The Computational Formulas for Sum of Squares: worked example
The Computational Formulas for Sum of Squares: worked example
The Computational Formulas for Sum of Squares: Summary
Mean Squares
NOTE: The value of the sum of squares becomes larger as variation increases. The sum of squares also increases with sample size.
Because of this, the SS cannot be viewed as a true measure of variation. Another measure of variation that we can use is the Mean Square.
• The mean square within groups describes the variability in
scores within the conditions of an experiment. It is symbolized
by MSW.
• The mean square between groups describes the differences
between the means of the conditions in a factor. It is
symbolized by MSB.
Coputation of Mean Squares
• Between • Within
MSwithin
between
SSbetween
between df
MS
MSbetween = between group mean
square
SSbetween = between group sum of
squares
dfbetween = between group degrees of
freedom
within
SSwithin df
MSwithin = within group mean square
SSwithin = within group sum of
squares
dfwithin = within group degrees of
freedom
Degrees of Freedom
Use the following equations to obtain the correct degrees of freedom:
k df within N total
k 1 df be twe e n
k = number of groups
Critical Value of F (F critical)
The Critical value of F (Fcrit) depends on:
• The degrees of freedom (both the dfbn = k – 1 and the dfwn
= N – k)
• The selected
To obtain the Fcrit from the F statistical table:
Use the dfB (the numerator) across the top of the table.
Use the dfW (the denominator) along the side of the
table.
Worked example of Mean
Square Computation Calculating the Mean Square using Table 8.2 data in the previous example
Computing Fobs
The analysis of variance yields an F ratio.
The F ratio is the variance between groups and variation within groups compared.
The larger our calculated F ratio, the increased likelihood that we
will have a statistically significant result.
M S within ( wn )
M S be twe e n (bn )
F obs
• Illustration of another way
of computing the Sum of Squares and Mean Squares
using the mean method
Dr. Richard Boateng, UGBS Slide 28
Example: does family size vary by religious affiliation?
Step 1: Find the mean for each sample
Step 2:Cal. (1) Sum of scores, (2) sum of sq. scores, (3) number of subjs., (4) and mean
computations
computations
computation
DECISION
•To reject the null hypothesis at the .05 significance level with 2 and 12 degrees of freedom, our calculated F ratio
must exceed 3.88.
•From the computation, our obtained F ratio of 8.24, is
clearly greater than the F critical, hence we must reject the null hypothesis.
•Interpretation: At 0.05 significant level, it is indeed true
that Family size does vary by religion.
Post Hoc Comparisons
• When the F-test is significant, we perform post hoc comparisons
• Post hoc comparisons are like t-tests
• We compare all possible pairs of level means
from a factor, one pair at a time to
determine which means differ significantly
from each other
Examples: The protected t test method and
Fisher Least Significant (LSD) method
The Protected t Test method
The null hypothesis for comparing any pair of means tested with the formula:
and is
Mserror = MSw
where MSw is simply taken from the ANOVA results and n1
and n2 are the sizes of the two samples whose means we are
comparing. The computed value of t is referred to the t tables at α = 0.05 for a two-tailed test with
the degrees of freedom (df) associated with the MSw (= N - k) and a decision is taken as to whether or not Ho should be
rejected
X1 X 2 X1 X 2
1 2 n1 n2
1
1 MSerror MSerror
MSerror
t
n n
Fisher LSD (Least Significant Difference)
method
• Used when all the groups have equal sample sizes, i.e. n1=n2=n3
• Then the denominator of the protected t test becomes a constant for all pairwise comparisons. In such a situation, it becomes possible to determine
what least significant difference (LSD) between means is needed to reject Ho at any given level of significance.
=
Note that t here refers to the critical value of t with N-k df in a two-tailed test
X1 X 2 X1 X 2
1 2 n1 n2
1
1 MSerror MSerror MSerror
t
n n
Two- way ANOVA- overview
• We have learned how to test for the effects of independent variables considered one at a time.
• However, much of human behavior is determined
by the influence of several variables operating at the same time.
• Sometimes these variables combine to influence performance.
Two- way ANOVA
• We need to test for the independent and combined effects
of multiple variables on performance. We do this with a
Two- way ANOVA that asks:
(i)how different from each other are the means for levels
of Variable A?
(ii)how different from each other are the means for levels
of Variable B?
(iii)how different from each other are the means for the
treatment combinations produced by A and B
together?
Two way ANOVA
• The first two of those questions are questions about main
effects of the respective independent variables.
• The third question is about the interaction effect,
the
effect of the two variables considered simultaneously.
MAIN vs INTERACTION EFFECTS
• Main effect •
–
A main effect is the effect on
performance of one treatment variable considered in isolation
(ignoring other variables in the
study)
Slide 42
Interaction Effect
– an
interaction effect occurs
when the effect of one
variable is different across
levels of one or more other variables
Illustration
• In order to detect interaction effects, we must use ͞factorial͟ designs.
– In a factorial design each variable is tested at every level of
all of the other variables.
– Below represent two variables A and B both with two
levels A1,A2 and B1,B2 respectively. A1 A2
B1 i ii
B2 iii iv
Illustration
I.vs III
– Effect of B at level A1 of variable A
II.vs IV
– Effect of B at A2
• If these are different, then we say that A and B interact
ALTERNATIVELY
I vs II
– Effect of A at B1
III vs IV
– Effect of A at B2
• If these are different, then we say that A and B interact
Illustration
B1
B1
B2
B2
A1 A2 A1 A2
•In the graphs above, the effect of A varies at levels of B, and the effect of B varies at levels of A. How you say it is a
matter of preference (and your theory).
•In each case, the interaction is the whole pattern. No part of the graph shows the interaction. It can only be seen in
the entire pattern (here, all 4 data points).
Computation of F ratios in Two- Way ANOVA
• In a Two-Way ANOVA, three F ratios are computed:
• One F ratio is computed for the factor represented along the
rows;
• a second F ratio is computed for the factor represented
along the columns; and
• a third F ratio is computed for the interaction between the
factors represented along the rows and columns.
• The various F ratios are each referred to the F tables with
the appropriate degrees of freedom associated with each F
ratio under a specified decision rule and a decision is taken
as to whether or not Ho should be rejected in each case.
Slide 46