この節の作者: Danielle J. Navarro and David R. Foxcroft
Scatterplots
Scatterplots are a simple but effective tool for visualising the
relationship between two variables, like we saw with the figures in the
section on correlation (section Correlations). It’s this latter
application that we usually have in mind when we use the term “scatterplot”.
In this kind of plot each observation corresponds to one dot. The horizontal
location of the dot plots the value of the observation on one variable, and
the vertical location displays its value on the other variable. In many
situations you don’t really have a clear opinions about what the causal
relationship is (e.g., does A cause B, or does B cause A, or does some other
variable C control both A and B). If that’s the case, it doesn’t really
matter which variable you plot on the x-axis and which one you plot on the
y-axis. However, in many situations you do have a pretty strong idea which
variable you think is most likely to be causal, or at least you have some
suspicions in that direction. If so, then it’s conventional to plot the cause
variable on the x-axis, and the effect variable on the y-axis. With that in
mind, let’s look at how to draw scatterplots in jamovi, using the same
parenthood
data set that I used when introducing correlations.
Suppose my goal is to draw a scatterplot displaying the relationship between
the amount of sleep that I get (dani.sleep
) and how grumpy I am the next day
(dani.grump
). There are two different ways in which we can use jamovi to get
the plot that we’re after. The first way is to use the Plot
option under
the Regression
→ Correlation Matrix
button, giving us the output shown
in 図 113. Note that jamovi draws a line through the
points, we’ll come onto this a bit later in section What is a linear regression model?.
Plotting a scatterplot in this way also allow you to specify Densities for
variables
and this option adds a density curve showing how the data in each
variable is distributed.
The second way do to it is to use one of the jamovi add-on modules. This
module is called scatr
and you can install it by clicking on the large
+
icon in the top right of the jamovi screen, opening the jamovi
library, scrolling down until you find scatr
and clicking Install
.
When you have done this, you will find a new Scatterplot
command
available under the Exploration
button. This plot is a bit different
than the first way, see 図 114, but the important
information is the same.
More elaborate options
Often you will want to look at the relationships between several
variables at once, using a scatterplot matrix (in jamovi via the
Correlation Matrix
- Plot
command). Just add another variable, for
example baby.sleep
to the list of variables to be correlated, and
jamovi will create a scatterplot matrix for you, just like the one in
図 115.