### Initial setup

```
rm(list=ls())
library(ggplot2)
library(MASS)
library(effects)
ts=25
```

# Correlation

Example of scatter plot featuring a correlation of exactly
`r = 0.40`

.

It can be understood as a +1.00 *SD* increase in one variable
corresponding to an estimated +0.40 *SD* increase in the other
one.

Standardized regression coefficient in a linear model (continuous
predictor) can be interpreted in a similar fashion.

# Standardized Mean Difference

SMDs (e.g., Cohen’s d, possibly with Hedges’ correction for small
samples) are reported to compare group average values on a standardized
metric. Here is an example of
`Cohen's d = 1.50`

between “**population green**” and
“**population
purple**”.

Height is commonly used for offering examples to understand SMDs,
because it is an immediately visible, non-latent, more or less
normally-distributed trait. For example, from
Funder
and Ozer (2019):