In this video we'll take a look at the two way between subjects ANOVA. And the
two-way between subjects ANOVA has two factors or independent variables and it
has one dependent variable.
So this is an expansion of the one-way anova. Recall that the one-way anova had
one factor or one independent variable and it's called one way to indicate that
there's one independent variable. So now notice that we have the two-way anova to
indicate that there's two independent variables.
And in this example we're going to continue and expand upon our example
with the volume that we saw before, where in this case we're going to have a no
volume group and a high volume group. We won't have a low volume group so just no
volume or high volume that's going to be our first factor as you can see here.
Factor one is volume none and high and then a second factor is study method
where here for study method we have two types either spaced studying or cramming.
Now we all know what cramming is and we probably all engaged in it at one time
or another I know I have to admit that I have in my collegiate experience at some
point or another, at several points frankly, but I tried not to it was a last
resort but it did happen. So we're cramming and spaced now spaced studying
is really what we would like to do if we could have research indicates generally
speaking that spaced studying is more effective than cramming and what I mean
here by spaced let's just consider an example suppose that a person was
studying five hours for an exam and cramming they would study for the five
hours the night immediately before the exam
whereas spaced as one example of a spaced method of studying they could study
instead of five hours the night before they could study one hour each of five
nights prior to the exam. So in this example here
consider these two groups to be studying the same amount but cramming they do it
all at once right before the exam and spaced they spread it out evenly over a
number of days.
so that's our second independent variable and then our dependent variable
once again is exam scores so all these people took an exam and we have their
scores here in the exam scores column as you can see.
Now for this example when once again notice we have two independent variables
volume is our independent variable or factor one study method is our
independent variable or factor two and exam scores is our dependent variable.
Now we can test each of these independent variables separately and
that's in fact what will happen in the two-way anova and these are called tests
of main effects and I'll explain that more in a little bit but let's go ahead
and take a look at an example here so it does volume and by volume we mean none
vs high does volume have an impact on exam scores? So we can use the two-way anova
to test simply whether none
vs high volume is there a difference in exam scores between these two groups?
That's one of the tests will get with the anova.
The second test that we'll get with the anova is our test of main effect two
the second main effect and that could ask something like the following: does
the study method used spaced vs cramming have an impact on exam scores? So here
now we're working with this independent variable and we're comparing the spaced
group versus the cramming group is there a difference in our exam scores for
these two groups and we'll get that test in our anova as well and then finally we
don't only get the two tests that I just described for the tests of the two main
effects but we also get an additional test in the two-way anova and this test
is what really can make the two-way anova so valuable and that is a test of
an interaction effect.
And we could think of the interaction effect as one way is something like the
following does the impact of volume that is none vs high
on our exam scores depend on and that phrase is key, does it depend on the type
of study method used space versus cramming? So in other words when we're
looking at volume none vs high
should we also take into account the study method that the person had or is
the impact of volume totally independent or separate from what study method of
person used?
Now the interaction effect frankly speaking takes a little bit of practice
I think to get used to and we're just introducing it in this video here and
I'll give you a little more feel for it as we go through the video but we're
going to spend an entire separate video on the interaction really dissecting
what it is in more detail.