An award-winning collaborative, global project.

This wiki is no longer being updated. But it is still available for anyone who wants to make style decisions based on evidence and data.


Part of: Readability Guidelines

Usability evidence definition

Last updated: 9 April, 2020

The Readability Guidelines project is entirely based on usability evidence.

Definition of usability evidence

For the purposes of this project, usability evidence is:

  • an academic paper,

  • a published, formal usability study.

Google Scholar is often a springboard in the search for evidence.


Some evidence sources can only be accessed with a subscription, paid download or or academic email address.

If you find a study that is inaccessible in this way, try searching for it elsewhere, as sometimes authors publish their studies in several places. They may have their own website where they make their work openly available.

Trusted sources

We know these organisations carry out extensive first-hand usability research that they base their studies, style guide or design system on:

  1. Nielsen Norman Group

  2. UK Government Digital Service (GDS)

  3. Google's

Please add in the comments of this page any online documentation about the extent of usability evidence carried out by any other organisations. Hopefully we can add more trusted sources to our list.

Robust evidence

In our live topic discussions we ask how robust a paper or study is:

  • is there more recent study that disproves it?

  • have any biases been discovered about the study?

  • is it specifically concerned with the style point we are discussing?

  • does it refer to users with access challenges?

Date of evidence may be important

Currently on this wiki we cite evidence from as far back as 1969. When relevant, we would ideally find more recent study that takes into account changes to readability that may be caused by digital advances.

In the live discussion we ask if anyone found a more recent study confirming or refuting findings, if a study is several decades old.

Blogs and articles are not considered evidence

We do not count blog posts and articles as evidence in themselves. However they may link to usability studies. We can cite the studies, if they are formal and robust.

The only exception would be pieces written by study authors, relating directly to their own study, or studies, where we find the study relevant and robust.