Readability Formulas and Writing for the Web

by Dr. Bob Bailey

July, 2002

Introduction

There is a considerable amount of information published on the Web that is intended to be read by someone. There is evidence that much of the information may be too hard to read and understand for typical readers.

Baker, Wilson and Kars (1997) reported that the readability scores of most articles in the 'Health Reference Center' ranged from 10th to 14th grade levels. Another study (Graber, Roller and Kaeble, 1999) included text-based information from commercial, academic and government sites. They found that the reading material averaged the 10th grade level. In a more recent study, a group of researchers (D'Alessandro, et.al., 2001) conducted readability analyses of pediatric patient education materials on the Web, and concluded that the information was not written at an appropriate reading level for typical users.


Readability Formulas

Readability formulas have been developed to assist writers in preparing information. These formulas provide a means for estimating the difficulty a reader may have reading and understanding a paragraph, section or entire document on the Web.

The first readability formula was developed over 80 years ago, and a number of formulas have been developed since that time. These formulas originally were designed to help classroom teachers choose textbooks for their students. Currently available, computer-based readability formulas include:

—Automated Readability Index
—Dale-Chall—Flesch Reading Ease (included with Microsoft Word)
—Flesch-Kincaid Grade Level (included with Microsoft Word)
—FOG
—SMOG (Simple Measure of Gobbledygook)

Readability results will vary depending on which formula is used. For example, the Flesch-Kincaid tool often returns a score two to three grades lower than other formulas. Osborne (2000) proposed that grade level equivalent scores tend to be accurate only by plus or minus 1.5 grade levels.

There are numerous factors that affect how easy, or how hard, a given document is to read and understand, including sentence length, word choice, layout and formatting, overall organization of the content, use of illustrations, etc. However, most readability formulas consider only two factors:
(a) the number of syllables (or letters) in a word, and
(b) the number of words in a sentence.

Because most readability formulas consider only these two factors, these formulas do not reveal why written material is difficult to read and comprehend. Most of the important attributes of writing that contribute to reading difficulty have not yet been quantified. Fortunately, many of the difficult-to-measure attributes are highly correlated with the two factors that can be easily measured.

Readability formulas are most useful as predictors of reading difficulty. Klare (1975), in a review of readability formulas, concluded that "as long as predictions are all that is needed, the evidence that simple word and sentence counts can provide satisfactory predictions for most purposes is now quite conclusive."

A document classified as highly readable solely on the basis of a readability formula could be a disorganized disaster—or contain no content at all. The following paragraph has a calculated readability of the 12th grade:

Obviously, readability scores depend on the writing style rather than the content of written material. These stylistic features are under the control of the writer.


Reading Skill of the Intended Audience

As general rule, it is better to write a document at a readability level that is below the reading skill level of the intended audience. Ideally, the reading skill level of intended readers would be based on the results of a standardized reading test (e.g., the Nelson-Denny Reading Test). This is usually reported as a grade level, i.e., "95% of the users in the target audience read at an 8th grade level or higher."

Usually it is not practical to administer a reading test to all potential users. An estimate of the reading grade level of the intended audience can be obtained by considering the users' education level. An average eighth grader is assumed to read at an 8th grade reading level, and a twelfth grader at a 12th grade level. People who have completed college are assumed to read at the 16th grade level.

In general, people with more education have better reading skills than people with less education. However, the actual reading ability of a person does not always match his or her educational level. Coke and Koether (1979) collected reading scores for over 200 company employees. The group averaged a 12th grade education, and 95% had reading test scores above the 10th grade reading level. Hilts and Krilyk (1991) reported that adults read at least one or two grade levels below their last school grade completed.

Summarizing several studies done in the United States and Canada, the average reading skill level was estimated to be at around the 8th to 9th grade (University of Utah Health Sciences Center). However, this same study found that about one in five adults had a reading skill level at the 5th grade level or below.


Conclusion

By comparing the calculated readability of a document to the reading skill level of typical users, a writer can estimate whether a document has a good chance of being read and understood. The readability formula can be used as a predictor of difficulty, but should not be used as a diagnostic tool. Readability formulas do not provide information about how to make instructions more comprehensible. For example, a document with a high readability level might be made more readable by changing its format rather than its writing style.

To make written texts truly readable, Website designers should apply all the principles of clear and simple writing. Even though using short words and short sentences will result in lower readability scores, this does not guarantee that a document will be easier to read.

Finally, there are times when document readability issues are not as important as other issues. Klare (1975) found that in circumstances where time is not crucial and readers are highly motivated, the readability of a document was of less importance. Coke (1976) provided evidence that readability was not as important when readers were looking for specific information as it was when users had to remember that information.

Incidentally, the Flesch-Kincaid readability level for this article is 12th grade.


References

Baker, L.M., Wilson, F.L. and Kars, M. (1997), The readability of medical information on InfoTrac: Does it meet the needs of people with low literacy skills? Reference & User Services Quarterly, Winter, 37(2), 155-160.

Coke, E.U. (1976), Reading rate, readability and variations in task?induced processing, Journal of Educational Psychology, 68, 167-173.

Coke, E.U. and Koether, M.E. (1979), The reading skills of craft and technical management employees: Estimates from two samples of students, Bell Laboratories Technical Report, May.

D'Alessandro, D., Kingsley, P. and Johnson, J. (2001), The readability of pediatric patient education materials on the Web, Archives of Pediatrics & Adolescent Medicine, July.

Graber, M.A., Roller, C.M. and Kaeble, B. (1999), Readability levels of patient education material on the World Wide Web, The Journal of Family Practice, January, 48(1), 58-61.

Hilts, L. and Krilyk B. J. (1991), Write Readable Information to Educate, Hamilton, Ontario: Chedoke-McMaster Hospitals and Hamilton Civic Hospitals.

Klare, G. R. (1975), Assessing readability, Reading Research Quarterly, 10, 62-102.

Osborne, H. (2000), Assessing readability: Rules for playing the numbers game, On Call, December.

University of Utah Health Sciences Center, Patient Education Materials: An Author's Guide.