Tag: groups

Visualizing and interpreting Cohen’s d effect sizes

Visualizing and interpreting Cohen’s d effect sizes

Cohen’s d (wiki) is a statistic used to indicate the standardised difference between two means. Resarchers often use it to compare the averages between groups, for instance to determine that there are higher outcomes values in a experimental group than in a control group.

Researchers often use general guidelines to determine the size of an effect. Looking at Cohen’s d, psychologists often consider effects to be small when Cohen’s d is between 0.2 or 0.3, medium effects (whatever that may mean) are assumed for values around 0.5, and values of Cohen’s d larger than 0.8 would depict large effects (e.g., University of Bath).

The two groups’ distributions belonging to small, medium, and large effects visualized

Kristoffer Magnusson hosts this Cohen’s d effect size comparison tool on his website the R Psychologist, but recently updated the visualization and its interactivity. And the tool looks better than ever:

Moreover, Kristoffer adds some nice explanatons of the numbers and their interpretation in real life situations:

If you find the tool useful, please consider buying Kristoffer a coffee or buying one of his beautiful posters, like the one above, or below:

Frequentisme betekenis testen poster horizontaal image 0

By the way, Kristoffer hosts many other interesting visualization tools (most made with JavaScript’s D3 library) on statistics and statistical phenomena on his website, have a look!

A Visual Introduction to Hierarchical Models, by Michael Freeman

Hierarchical models I have covered before on this blog. These models are super relevant in practice. For instance, in HR, employee data is always nested within teams which are in turn nested within organizational units. Also in my current field of insurances, claims are always nested within policies, which can in turn be nested within product categories. Data is hierachical, and we need to take that into account when we model it.

Hierarchical models do just that. Interested in how they do this? Have a look at this amazing browser application made in React.js!


This project was built by Michael Freeman, a faculty member at the University of Washington Information School.

All code for this project is on GitHub, including the script to create the data and run regressions (done inR). Feel free to issue a pull request for improvements, and if you like it, share it on Twitter. Layout inspired by Tony Chu.

About this project
What to consider when choosing colors for data visualization, by DataWrapper.de

What to consider when choosing colors for data visualization, by DataWrapper.de

Lisa Charlotte Rost of DataWrapper often writes about data visualization and lately she has focused on the (im)proper use of color in visualization. In this recent blog, she gives a bunch of great tips and best practices, some of which I copied below:

color in data vis advice
Gradient colors can be great to show a pattern but, for categorical data, it is often easier to highlight the most important values with colored bars, positions (like in a dot plot) or even areas. [https://blog.datawrapper.de/colors/]
color in data vis advice
If you need more than seven colors in a chart, consider using another chart type or to group categories together. [https://blog.datawrapper.de/colors/]
color in data vis advice
Consider using the same color for the same variables, but do differentiate between categories, even across graphics. [https://blog.datawrapper.de/colors/]
color in data vis advice
Using grey for less important elements in your chart makes your highlight colors (which should be reserved for your most important data points) stick out even more.  [https://blog.datawrapper.de/colors/]
color in data vis advice
Consider color-blind people. There are many different types of color blindness: Use an online tool or Datawrapper’s automatic colorblind-check. [https://blog.datawrapper.de/colors/]
 You can find additional useful tips in the original DataWrapper blog.

Robust Effect Sizes for Independent Group Comparisons

Robust Effect Sizes for Independent Group Comparisons

Guillaume Rousselet explains how and when group comparisons with Cohen’s d fail, and what robust statistics one could use instead:

basic statistics

When I was an undergrad, I was told that beyond a certain sample size (n=30 if I recall correctly), t-tests and ANOVAs are fine. This was a lie. I wished I had been taught robust methods and that t-tests and ANOVAs on means are only a few options among many alternatives. Indeed, t-tests and ANOVAs on means are not robust to outliers, skewness, heavy-tails, and for independent groups, differences in skewness, variance (heteroscedasticity) and combinations of these factors (Wilcox & Keselman, 2003; Wilcox, 2012). The main consequence is a lack of statistical power. For this reason, it is often advised to report a measure of effect size to determine, for instance, if a non-significant effect (based on some arbitrary p value threshold) could be due to lack of power, or reflect a genuine lack of effect. The rationale is that an effect could be associated with a sufficiently large effect…

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