How do we know? (The evidence)

Scientific evidence and research supporting these readability principles.

Readability Research

Readability is a difficult subject to get reliable data on. It is difficult to define exactly what a readable text is and how to measure it. Various measures have been introduced:

Speed of reading

Comprehension

Subjective perception

Text interaction

But each of these has many confounders. Because no unified measures exist, there are few systematic reviews. However, many studies generally point in the same direction.

Evidence for text formatting

The key interventions supported by evidence are:

Font size (Rello and Baeza-Yates 2017; Rello, Pielot, and Marcos 2016; Simas da Rocha et al. 2020)
Line length (Nanavati and Bias 2005)
Bolding of key words (Rello, Kanvinde, and Baeza-Yates 2012)
Using colours (Simas da Rocha et al. 2020; Jandreau, Muncer, and Bever 1986)

Evidence for text structure

Structured text is easier to read (Kendeou and van den Broek 2007; Schwarz and Flammer 1981)
Eye tracking studies support this Well supported by eye tracking studies on web reading by Norman Nielsen Group.

Evidence for simple language

Using frequent and short words (Rello et al. 2013; Dale and Chall 1949)
Limits of readability formulas (Bruce, Rubin, and Starr 1981)

Evidence for reading strategies

Strategic reading is one of the key recommendations for improving academic reading. There is good evidence that it improves comprehension and information retention.

(Voss and Silfies 1996; Kendeou and van den Broek 2007; Bogaerds-Hazenberg, Evers-Vermeul, and Bergh 2021)

References

Bogaerds-Hazenberg, Suzanne T. M., Jacqueline Evers-Vermeul, and Huub van den Bergh. 2021. ‘A Meta-Analysis on the Effects of Text Structure Instruction on Reading Comprehension in the Upper Elementary Grades’. Reading Research Quarterly 56 (3): 435–62. https://doi.org/10.1002/rrq.311
Bruce, Bertram, Andee Rubin, and Kathleen Starr. 1981. ‘Why Readability Formulas Fail’. IEEE Transactions on Professional Communication PC-24 (1): 50–52. https://doi.org/10.1109/TPC.1981.6447826
Dale, Edgar, and Jeanne S. Chall. 1949. ‘The Concept of Readability’. Elementary English 26 (1): 19–26.
Jandreau, Steven M, Steven J Muncer, and Thomas G Bever. 1986. ‘Improving the Readability of Text with Automatic Phrase-Sensitive Formating’. British Journal of Educational Technology 17 (2): 128–33. https://doi.org/10.1111/j.1467-8535.1986.tb00502.x
Kendeou, Panayiota, and Paul van den Broek. 2007. ‘The Effects of Prior Knowledge and Text Structure on Comprehension Processes during Reading of Scientific Texts’. Memory & Cognition 35 (7): 1567–77. https://doi.org/10.3758/BF03193491
Nanavati, Anuj A., and Randolph G. Bias. 2005. ‘Optimal Line Length in Reading - A Literature Review’. Visible Language 39 (2): 120–44.
Rello, Luz, and Ricardo Baeza-Yates. 2017. ‘How to Present More Readable Text for People with Dyslexia’. Universal Access in the Information Society 16 (1): 29–49.
Rello, Luz, Ricardo Baeza-Yates, Laura Dempere-Marco, and Horacio Saggion. 2013. ‘Frequent Words Improve Readability and Short Words Improve Understandability for People with Dyslexia’. In IFIP Conference on Human-Computer Interaction, 203–19. Springer.
Rello, Luz, Gaurang Kanvinde, and Ricardo Baeza-Yates. 2012. ‘Layout Guidelines for Web Text and a Web Service to Improve Accessibility for Dyslexics’. In Proceedings of the International Cross-Disciplinary Conference on Web Accessibility, 1–9.
Rello, Luz, Martin Pielot, and Mari-Carmen Marcos. 2016. ‘Make It Big! The Effect of Font Size and Line Spacing on Online Readability’. In Proceedings of the 2016 CHI Conference on Human Factors in Computing Systems, 3637–48.
Schwarz, Maria N. K., and August Flammer. 1981. ‘Text Structure and Title—Effects on Comprehension and Recall’. Journal of Verbal Learning and Verbal Behavior 20 (1): 61–66. https://doi.org/10.1016/S0022-5371(81)90301-7
Simas da Rocha, Bruno, Cassia Garcia Moraes, Lucas Miyake Okumura, Fernanda da Cruz, Lisana Sirtori, and Emilia da Silva Pons. 2020. ‘Interventions to Reduce Problems Related to the Readability and Comprehensibility of Drug Packages and Labels: A Systematic Review.’ Journal of Patient Safety, April. https://doi.org/10.1097/pts.0000000000000619
Voss, James F., and Laurie Ney Silfies. 1996. ‘Learning From History Text: The Interaction of Knowledge and Comprehension Skill with Text Structure’. Cognition and Instruction 14 (1): 45–68. https://doi.org/10.1207/s1532690xci1401_2