This vignette can be referred to by citing the package:

- Makowski, D., Ben-Shachar, M. S., & Lüdecke, D. (2019).
*bayestestR: Describing Effects and their Uncertainty, Existence and Significance within the Bayesian Framework*. Journal of Open Source Software, 4(40), 1541. https://doi.org/10.21105/joss.01541

Now that **describing
and understanding posterior distributions** of linear
regressions is not that mysterious to you, we will take one step back
and study some simpler models: **correlations** and
** t-tests**.

But before we do that, let us take a moment to remind ourselves and
appreciate the fact that **all basic statistical
procedures** such as correlations, *t*-tests, ANOVAs, or
chi-square tests **are** linear regressions (we strongly
recommend this
excellent demonstration). Nevertheless, these simple models will
provide a good pretext to introduce a few more complex indices, such as
the **Bayes factor**.

## Correlations

### Frequentist version

Once again, let us begin with a **frequentist
correlation** between two continuous variables, the
**width** and the **length** of the sepals of
some flowers. The data is available in `R`

as the
`iris`

dataset (the same that was used in the previous
tutorial).

We will compute a Pearson’s correlation test, store the results in an
object called `result`

, and then display it:

```
result <- cor.test(iris$Sepal.Width, iris$Sepal.Length)
result
```

```
>
> Pearson's product-moment correlation
>
> data: iris$Sepal.Width and iris$Sepal.Length
> t = -1, df = 148, p-value = 0.2
> alternative hypothesis: true correlation is not equal to 0
> 95 percent confidence interval:
> -0.273 0.044
> sample estimates:
> cor
> -0.12
```

As you can see in the output, the test actually compared
**two** hypotheses: - the **null hypothesis**
(*h0*; no correlation), - the **alternative
hypothesis** (*h1*; a non-null correlation).

Based on the *p*-value, the null hypothesis cannot be
rejected: the correlation between the two variables is **negative
but non-significant** (\(r = -.12, p
> .05\)).

### Bayesian correlation

To compute a Bayesian correlation test, we will need the `BayesFactor`

package (you can install it by running
`install.packages("BayesFactor")`

). We can then load this
package, compute the correlation using the `correlationBF()`

function, and store the result.

```
library(BayesFactor)
result <- correlationBF(iris$Sepal.Width, iris$Sepal.Length)
```

Now, let us run our `describe_posterior()`

function on
that:

`describe_posterior(result)`

```
> Summary of Posterior Distribution
>
> Parameter | Median | 95% CI | pd | ROPE | % in ROPE | BF | Prior
> -----------------------------------------------------------------------------------------------
> rho | -0.11 | [-0.27, 0.04] | 92.25% | [-0.05, 0.05] | 20.42% | 0.509 | Beta (3 +- 3)
```

We see again many things here, but the important indices for now are
the **median** of the posterior distribution,
`-.11`

. This is (again) quite close to the frequentist
correlation. We could, as previously, describe the **credible
interval**, the **pd**
or the **ROPE
percentage**, but we will focus here on another index
provided by the Bayesian framework, the **Bayes Factor
(BF)**.

### Bayes Factor (BF)

We said previously that a correlation test actually compares two
hypotheses, a null (absence of effect) with an alternative one (presence
of an effect). The **Bayes
factor (BF)** allows the same comparison and determines
**under which of these two models the observed data are more
probable**: a model with the effect of interest, and a null model
without the effect of interest. So, in the context of our correlation
example, the null hypothesis would be no correlation between the two
variables (\(h0: \rho = 0\); where
\(\rho\) stands for Bayesian
correlation coefficient), while the alternative hypothesis would be that
there is a correlation **different** than 0 - positive or
negative (\(h1: \rho \neq 0\)).

We can use `bayesfactor_models()`

to specifically compute
the Bayes factor comparing those models:

`bayesfactor_models(result)`

```
> Bayes Factors for Model Comparison
>
> Model BF
> [2] (rho != 0) 0.509
>
> * Against Denominator: [1] (rho = 0)
> * Bayes Factor Type: JZS (BayesFactor)
```

We got a *BF* of `0.51`

. What does it mean?

Bayes factors are **continuous measures of relative
evidence**, with a Bayes factor greater than 1 giving evidence in
favour of one of the models (often referred to as

*the numerator*), and a Bayes factor smaller than 1 giving evidence in favour of the other model (

*the denominator*).

Yes, you heard that right, evidence in favour of thenull!

That’s one of the reason why the Bayesian framework is sometimes
considered as superior to the frequentist framework. Remember from your
stats lessons, that the ** p-value can only be used to
reject h0**, but not

*accept*it. With the

**Bayes factor**, you can measure

**evidence against - and in favour of - the null**. In other words, in the frequentist framework, if the

*p*-value is not significant, we can conclude that

**evidence for the effect is absent**, but not that there is

**evidence for the absence of the effect**. In Bayesian framework, we can do the latter. This is important since sometimes our hypotheses are about no effect.

BFs representing evidence for the alternative against the null can be
reversed using \(BF_{01}=1/BF_{10}\)
(the *01* and *10* correspond to *h0* against
*h1* and *h1* against *h0*, respectively) to
provide evidence of the null against the alternative. This improves
human readability^{1} in cases where the BF of the alternative
against the null is smaller than 1 (i.e., in support of the null).

In our case, `BF = 1/0.51 = 2`

, indicates that the data
are **2 times more probable under the null compared to the
alternative hypothesis**, which, though favouring the null, is
considered only anecdotal
evidence against the null.

We can thus conclude that there is **anecdotal evidence in
favour of an absence of correlation between the two variables
(r _{median} = 0.11, BF = 0.51)**, which is a much more
informative statement that what we can do with frequentist
statistics.

**And that’s not all!**

### Visualise the Bayes factor

In general, **pie charts are an absolute no-go in data
visualisation**, as our brain’s perceptive system heavily
distorts the information presented in such way^{2}. Nevertheless, there
is one exception: pizza charts.

It is an intuitive way of interpreting the strength of evidence provided by BFs as an amount of surprise.

Such “pizza plots” can be directly created through the `see`

visualisation companion package for `easystats`

(you can
install it by running `install.packages("see")`

):

```
library(see)
plot(bayesfactor_models(result)) +
scale_fill_pizza()
```

So, after seeing this pizza, how much would you be surprised by the outcome of a blinded poke?

##
*t*-tests

“I know that I know nothing, and especially not if.versicolorandvirginicadiffer in terms of their Sepal.Width” - Socrates

Time to finally answer this crucial question!

### Versicolor *vs.* virginica

Bayesian *t*-tests can be performed in a very similar way to
correlations. As we are particularly interested in two levels of the
`Species`

factor, *versicolor* and *virginica*.
We will start by filtering out from `iris`

the non-relevant
observations corresponding to the *setosa* specie, and we will
then visualise the observations and the distribution of the
`Sepal.Width`

variable.

```
library(datawizard)
library(ggplot2)
# Select only two relevant species
data <- droplevels(data_filter(iris, Species != "setosa"))
# Visualise distributions and observations
ggplot(data, aes(x = Species, y = Sepal.Width, fill = Species)) +
geom_violindot(fill_dots = "black", size_dots = 1) +
scale_fill_material() +
theme_modern()
```

It *seems* (visually) that *virgnica* flowers have, on
average, a slightly higer width of sepals. Let’s assess this difference
statistically by using the `ttestBF()`

function in the
`BayesFactor`

package.

### Compute the Bayesian *t*-test

```
result <- BayesFactor::ttestBF(formula = Sepal.Width ~ Species, data = data)
describe_posterior(result)
```

```
> Summary of Posterior Distribution
>
> Parameter | Median | 95% CI | pd | ROPE | % in ROPE | BF | Prior
> ------------------------------------------------------------------------------------------------------
> Difference | -0.19 | [-0.32, -0.06] | 99.75% | [-0.03, 0.03] | 0% | 17.72 | Cauchy (0 +- 0.71)
```

From the indices, we can say that the difference of
`Sepal.Width`

between *virginica* and
*versicolor* has a probability of **100% of being
negative** [*from the pd and the sign of the median*]
(median = -0.19, 89% CI [-0.29, -0.092]). The data provides a
**strong evidence against the null hypothesis** (BF =
18).

Keep that in mind as we will see another way of investigating this question.

## Logistic Model

A hypothesis for which one uses a *t*-test can also be tested
using a binomial model (*e.g.*, a **logistic
model**). Indeed, it is possible to reformulate the following
hypothesis, “*there is an important difference in this variable
between the two groups*” with the hypothesis “*this variable is
able to discriminate between (or classify) the two groups*”.
However, these models are much more powerful than a *t*-test.

In the case of the difference of `Sepal.Width`

between
*virginica* and *versicolor*, the question becomes,
*how well can we classify the two species using only*
`Sepal.Width`

.

### Visualise the model

Using the `modelbased`

package.

```
library(modelbased)
vizdata <- estimate_relation(model)
ggplot(vizdata, aes(x = Sepal.Width, y = Predicted)) +
geom_ribbon(aes(ymin = CI_low, ymax = CI_high), alpha = 0.5) +
geom_line() +
ylab("Probability of being virginica") +
theme_modern()
```

### Performance and Parameters

Once again, we can extract all indices of interest for the posterior
distribution using our old pal `describe_posterior()`

.

`describe_posterior(model, test = c("pd", "ROPE", "BF"))`

```
> Summary of Posterior Distribution
>
> Parameter | Median | 95% CI | pd | ROPE | % in ROPE | BF | Rhat | ESS
> ------------------------------------------------------------------------------------------------------
> (Intercept) | -6.12 | [-10.45, -2.25] | 99.92% | [-0.18, 0.18] | 0% | 12.91 | 1.000 | 26540.00
> Sepal.Width | 2.13 | [ 0.79, 3.63] | 99.94% | [-0.18, 0.18] | 0% | 14.63 | 1.000 | 26693.00
```

```
library(performance)
model_performance(model)
```

```
> # Indices of model performance
>
> ELPD | ELPD_SE | LOOIC | LOOIC_SE | WAIC | R2 | RMSE | Sigma | Log_loss | Score_log | Score_spherical
> -----------------------------------------------------------------------------------------------------------------
> -66.284 | 3.052 | 132.568 | 6.104 | 132.562 | 0.099 | 0.477 | 1.000 | 0.643 | -35.436 | 0.014
```