Another good way to use bar graphs is with repeated measures. If we measure the
same people multiple times, we can see whether their average scores gradually
increased over time, decreased over time stayed the same, or perhaps started low
and then got high. We can see those patterns with a bar chart. In this
example, we are going to graph the level of participation across three trials in
the clickers.sav dataset. We are going to graph the level of participation across
three rounds of seminar group discussion interactions. The scores for each round
or trial will go on the x-axis. The mean, on a scale of one to seven, will be
plotted on the y-axis. These scores will be treated as scale data. Now because we
are measuring the same people three times, this is called a repeated measure.
Repeated measures are also called dependent measures. As a participant, your
score on the third round somewhat depends on your score from the first
round. Level of participation at a table, i.e. within a group, can be influenced by a
person, or by others in the group. By contrast, if scores in one group do not
affect scores in the next group, then those scores are independent. When we are
comparing the individual clicker group to the shared clicker group, that is an
independent measure. Being a male in one group has no influence on the gender of
someone at another table. In the same way, measures of participation between groups
are independent. Level of participation at one table or within one group has no
influence on participation at another table. To display the bar chart for
repeated measures, we will use the clickers.sav data set, and the chart
builder in SPSS. To display a bar chart for repeated-measures, we
will use the clickers.sav data set and the chart builder in SPSS. Go to graphs >
chart builder. If there is a previous analysis in the chart builder, you can
click "reset" to restore the chart builder to its pristine state. Now here in the
gallery, choose bar, and then drag simple bar chart up into the canvas. Remember
that you can right-click or control-click on any of the variable
names to toggle between, show variable names, and show variable labels. If you
are unsure about what an icon means, you can hover over that icon with your
cursor and the name will appear in a yellow box. As you make changes, the view
here in the canvas will not look like other students, or the example that I'm
showing you on screen. The final bar chart output however, will look identical.
Under variables, I'm going to make sure that I'm showing the variable names now
click on round 1, and holding down the shift key click on round 3. All three
variables are selected. Without letting off the shift key, click on any of those
three variables and drag all three simultaneously into the y-axis drop zone,
until you see the red plus sign. You can let off the shift key, and then click OK
to accept the resulting dialog box called create summary group. Now in newer
versions of SPSS, you can drag and drop the variables one at a time into this y
axis. If it is not open already, click on element properties. In this case, the
element property window is already open. In this area, labeled "edit properties of"
bar one has already been selected. We can see that the statistic for bar one is
round 1:mean. Make sure that the bar style is set
to bar, so there's nothing to change here. Now click on axis1 bar1. In the
axis label field, type "Round of Group Interaction" and now click apply. Click on
y-axis1 bar 1, change the name of the axis label to mean participation, and
then click apply, and okay. Scroll down a little to see the bar chart, and now we
can take a closer look at it.
Here is our repeated-measures bar chart. We see that round one has the tallest
bar, and that rounds two and three are very similar to each other. Right now, it
seems like participation was higher during round one, but there may be more
to the picture than first appears. To get a better picture of our data however, we
should add error bars to our repeated-measures bar chart. Now the
error bar looks like a little I beam that sticks out of the top of the main
bar. It tells us something about how well the top of the bar represents the mean.