Table 1.

Summary of commonly used readability formulas [22, 31]

Instrument	Variables	Advantages	Disadvantages	Comments
Flesch Reading Ease score	Average number of syllables per word and average number of words per sentence. Flesch Reading Ease score = 206.835 − 0.846 × number of syllables per 100 words − 1.015 × average number of words per sentence	Widely available through Microsoft® Word® Office package	Interpretation is relatively difficult as score ranges from 0–100 and negatively correlates with other readability formulas	As FRE scores increase, the FK Grade level decreases and vice versa
Flesch Kincaid Grade	Same as FRE score. Flesch-Kincaid Grade = 0.39 × average number of words per sentence + 11.8 × number of syllables per word – 15.59	Wide availability, extensively validated, quick and easy to administer, correlates highly with other readability formulas	Solely based on polysyllable words and long sentences, thus may underestimate reading difficulty of medical jargon that may contain short but unfamiliar words	Most commonly used formula; available only for English language
Gunning Fog Index	Average number of words per sentence, percentage of words with more than 2 syllables. Gunning Fog index = 0.4 × (average number of words per sentence + percentage of words with more than two syllables)	Needs less time to administer if done manually compared to FRE and FK Grade formula	Computer software measuring index is not as widely available as FRE and FK Grade	Like FRE, FK grade, SMOG and Fry, Fog Index also cannot measure reading difficulty of text arranged in tables and rows
SMOG Readability formula	Number of words with more than 2 syllables. SMOG score = 3 + square root of polysyllable count (number of words with more than two syllables) in 30 sentences.	Compared to other formulas, SMOG uses a stricter criterion, ie, it aims for 100% comprehension; thus, the grade level readability will be usually about 1–2 grades higher than FK Grade	Not as widely available as FRE and FK Grade	Recommended by National Cancer Institute for cancer pamphlets
Fry Readability Graph	Number of sentences and syllables per 100 words.	Validated in both Spanish and English; correlates highly with other readability formulas	Software tools that can estimate graph not as widely available as FRE and FK Grade	Recommended by some because of the ease of manual administration compared to other readability tools
The New Dale-Chall readability formula	Number of syllables per sentence and percentage of difficult words. raw score = 0.0496 × average words per sentence + 0.1579 × % unfamiliar words + 3.6365	Validated for comprehension; takes into consideration the vocabulary and sentence structure	Not available in commonly available word processing software	Developed primarily for health education materials
Suitability Assessment of Materials	22 different variables	Can be used for both print media and audiovisual tools; only tool that can assess the influence of illustrations on comprehension	Time consuming, steep learning curve; not as extensively validated as other readability tools, subjective; no software tools available and has to be administered manually

FRE = Flesch Reading Ease score; FK Grade = Flesch-Kincaid Grade; SAM = Suitability Assessment of Materials.

Adapted from Friedman DB, Hoffman-Goetz L. A systematic review of readability and comprehension instruments used for print and web-based cancer information. Health Educ Behav. 2006;33:352–373 ©2006 Sage Publications [22] and reprinted by permission of SAGE Publications.