In this section, having previously created a module, we are going to add a t-test analysis to it.
An independent samples t-test is typically used to determine whether two groups differ on some variable. For example, you may be interested in whether children from one neighbourhood are a different height than children from another neighbourhood. In this case, a data set would have one row for each child - one column would contain their heights, the other column would contain which neighbourhood they are from.
For those familiar with the
t.test function built into R, if the columns were
neighbourhood, the test could be run:
t.test(height ~ neighbourhood, data=data)
In this next section, we are going to create a t-test jamovi analysis which makes use of this function.
The easiest way to begin an analysis for a jamovi module is to use the
addAnalysis() function from the
jmvtools package. First we’ll set the working directory to the
SuperAwesome directory (created by
jmvtools::create() in the previous section), and then call
setwd('SuperAwesome') jmvtools::addAnalysis(name='ttest', title='Independent Samples T-Test')
If we now look at the
SuperAwesome module’s source tree, we have:
SuperAwesome/ ├── DESCRIPTION ├── NAMESPACE ├── jamovi/ | ├── ttest.a.yaml | ├── ttest.r.yaml | ├── ttest.u.yaml | └── 0000.yaml └── R/ ├── ttest.h.R └── ttest.b.R
At this point I’m a little embarrassed to say that there are 5 files per analysis. This structure does make things a lot more manageable, so although it may seem excessive at first, there’s a good rationale behind it. Just bear with me, it’s not as overwhelming as it seems.
|ttest.a.yaml||The analysis definition contains all the meta information about the analysis, describes the arguments and ui options it receives, etc. We’ll cover this in the tutorial next.||API|
|ttest.r.yaml||The results definition contains all the information about the way the results for the analysis are displayed.||API|
|ttest.u.yaml||The ui definition is automatically generated from the
|ttest.h.R||The header file is automatically generated from the
|ttest.b.R||The analysis implementation is where you’ll write your analysis, and where you’ll spend most of your time when developing jamovi analyses. As we’ll see, this is an empty template waiting for an implementation.|
First up, we’ll look at the analysis definition in the
ttest.a.yaml file. yaml is a simple, easy to read text format for structured data. If you’re new to yaml, you can read more about the format here, but that probably won’t be necessary. It’s intuitive and readable, and you’ll probably have no trouble picking it up.
jmvtools::addAnalysis() creates a default
.a.yaml file with a handful of basic options. Happily for this tutorial, these happen to correspond to the options that a t-test requires. Our generated
ttest.a.yaml file is as follows:
--- name: ttest title: Independent Samples T-Test jas: "1.1" version: "1.0.0" options: - name: data type: Data - name: dep title: Dependent Variable type: Variable - name: group title: Grouping Variable type: Variable - name: alt title: Alternative hypothesis type: List options: - not equal - one greater - two greater default: not equal - name: varEq title: Assume equal variances type: Bool default: true
At the top of the file is information about the analysis:
--- name: ttest title: Independent Samples T-Test jas: "1.0" version: "1.0.0" ...
The name is a unique identifier for the analysis, but it also corresponds to the name of the R function which will be made available from the package. As such, the usual constraints on function names apply: it must begin with a letter, and can only be made up of letters and numbers (jamovi disallows analysis names with dots in them).
The title of the analysis is how the analysis appears in menus, help text, and in the results. Next is the
jas or ‘jamovi analysis spec’; you should just leave this at ‘1.1’. Next, we have the analysis version. Each analysis is versioned in jamovi, and you should make use of semantic versioning (but let’s not worry about that for now).
The options come next;
varEq. When used as an R package, these options correspond to the arguments passed to the analysis R function. In this case they will be:
ttestIS(data, dep, group, alt='not equal', varEq=FALSE)
(If you peek inside
ttest.h.R, you’ll find this exact function has been generated.)
When used in jamovi, these options correspond to the UI options for the analysis. Let’s look at that now. Install the module we’ve just created and open up jamovi (or open up jamovi first, then install the module — the order doesn’t matter):
(Note that if your module has additional dependencies, these will be installed by
jmvtools::install() at this stage as well. You can read more about additional dependencies in Additional Notes at the end of this tutorial.)
SuperAwesome menu will now be available, and will contain
Independent Samples T-Test.
Selecting this analysis will bring up the UI:
We can see how the UI elements created, correspond to the analyses’ options. Revisiting the
group options from
... - name: dep title: Dependent Variable type: Variable - name: group title: Grouping Variable type: Variable ...
We see they are of type
Variable. This means they require the user to provide a column from the data set. As can be seen in the UI, they are represented as ‘drop targets’, where columns from the data set can be assigned through drag and drop.
Turning to the option
... - name: alt title: Alternative hypothesis type: List options: - not equal - one greater - two greater default: not equal ...
alt is of type list, which has been turned into a drop down list, allowing the user to choose between the options.
Finally, looking at
... - name: varEq title: Assume equal variances type: Bool default: true
We see that options of type
Bool are represented as checkboxes.
There are a number of different option types. A more comprehensive description of each of them, and their properties is available in the API.
Of course, our analysis doesn’t actually do anything yet – we haven’t written any R code.