Factorial design - Dr. Manu Melwin Joy - School of Management Studies, Cochin University of Science and Technology

Statistical Methods for Engineering Research
Factorial design, 2n
factorial design –
22
and 23
factorial design

Prepared By
Dr. Manu Melwin Joy
Assistant Professor
School of Management Studies
Cochin University of Science and Technology
Kerala, India.
Phone – 9744551114
Mail – manumelwinjoy@cusat.ac.in
Kindly restrict the use of slides for personal purpose.
Please seek permission to reproduce the same in public forms and
presentations.

Factorial design
• In statistics, a full factorial experiment is an
experiment whose design consists of two or more
factors, each with discrete possible values or
"levels", and whose experimental units take on all
possible combinations of these levels across all
such factors.

Factorial design
• Probably the easiest way to begin understanding
factorial designs is by looking at an example. Let's
imagine a design where we have an educational
program where we would like to look at a variety
of program variations to see which works best. For
instance, we would like to vary the amount of time
the children receive instruction with one group
getting 1 hour of instruction per week and another
getting 4 hours per week.

Factorial design
• And, we'd like to vary the setting with one group
getting the instruction in-class (probably pulled
off into a corner of the classroom) and the other
group being pulled-out of the classroom for
instruction in another room. We could think about
having four separate groups to do this, but when
we are varying the amount of time in instruction,
what setting would we use: in-class or pull-out?
And, when we were studying setting, what amount
of instruction time would we use: 1 hour, 4 hours,
or something else?

Factorial design
• In factorial designs, a factor is a major
independent variable. In this example we have two
factors: time in instruction and setting. A level is a
subdivision of a factor. In this example, time in
instruction has two levels and setting has two
levels.

Factorial design
• Sometimes we depict a factorial design with a
numbering notation. In this example, we can say
that we have a 2 x 2 (spoken "two-by-two)
factorial design. In this notation, the number of
numbers tells you how many factors there are and
the number values tell you how many levels. If I
said I had a 3 x 4 factorial design, you would
know that I had 2 factors and that one factor had 3
levels while the other had 4.

Factorial design
• Now, let's look at a variety of different results we
might get from this simple 2 x 2 factorial design.
Each of the following figures describes a different
possible outcome. And each outcome is shown in
table form (the 2 x 2 table with the row and
column averages) and in graphic form (with each
factor taking a turn on the horizontal axis). You
should convince yourself that the information in
the tables agrees with the information in both of
the graphs. You should also convince yourself that
the pair of graphs in each figure show the exact
same information graphed in two different ways

The Null Outcome
• Let's begin by looking at the "null" case. The null
case is a situation where the treatments have no
effect. This figure assumes that even if we didn't
give the training we could expect that students
would score a 5 on average on the outcome test.
You can see in this hypothetical case that all four
groups score an average of 5 and therefore the row
and column averages must be 5. You can't see the
lines for both levels in the graphs because one line
falls right on top of the other.

The Main Effects
• A main effect is an outcome that is a consistent
difference between levels of a factor. For instance,
we would say there’s a main effect for setting if
we find a statistical difference between the
averages for the in-class and pull-out groups, at all
levels of time in instruction. The first figure
depicts a main effect of time. For all settings, the 4
hour/week condition worked better than the 1
hour/week one. It is also possible to have a main
effect for setting (and none for time).

Interaction Effects
• If we could only look at main effects, factorial designs
would be useful. But, because of the way we combine
levels in factorial designs, they also enable us to
examine the interaction effects that exist between
factors. An interaction effect exists when differences
on one factor depend on the level you are on another
factor. It's important to recognize that an interaction is
between factors, not levels. We wouldn't say there's an
interaction between 4 hours/week and in-class
treatment. Instead, we would say that there's an
interaction between time and setting, and then we
would go on to describe the specific levels involved.

Interaction Effects
• How do you know if there is an interaction in a factorial design? There
are three ways you can determine there's an interaction. First, when
you run the statistical analysis, the statistical table will report on all
main effects and interactions. Second, you know there's an interaction
when can't talk about effect on one factor without mentioning the other
factor. if you can say at the end of our study that time in instruction
makes a difference, then you know that you have a main effect and not
an interaction (because you did not have to mention the setting factor
when describing the results for time). On the other hand, when you
have an interaction it is impossible to describe your results accurately
without mentioning both factors. Finally, you can always spot an
interaction in the graphs of group means -- whenever there are lines
that are not parallel there is an interaction present! If you check out the
main effect graphs above, you will notice that all of the lines within a
graph are parallel. In contrast, for all of the interaction graphs, you
will see that the lines are not parallel.

Interaction Effects
• In the first interaction effect graph, we see that one
combination of levels -- 4 hours/week and in-class
setting -- does better than the other three. In the
second interaction we have a more complex
"cross-over" interaction. Here, at 1 hour/week the
pull-out group does better than the in-class group
while at 4 hours/week the reverse is true.
Furthermore, the both of these combinations of
levels do equally well.

Factorial design - Dr. Manu Melwin Joy - School of Management Studies, Cochin University of Science and Technology

Factorial design - Dr. Manu Melwin Joy - School of Management Studies, Cochin University of Science and Technology

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Factorial design - Dr. Manu Melwin Joy - School of Management Studies, Cochin University of Science and Technology