Readability refers to how easy a written text is to read. Generally, readability can be broken down into two factors: how well the text is laid out visually, and how easily the words and sentences can be understood. Good writing should be highly readable in order to be clearly understood by a wide audience.
The first characteristic of readability pertains to the visual layout of words on a page or screen. The visual readability of a text is effected by the typeset, the length of lines, word spacing, and other visual factors. Researchers use several methods to test readability, generally involving human participants rather than mathematical formulas. These tests may measure how well the participant is able to read the material in a short period of time, from a distance, or through peripheral vision. Other methods include recording the eye movements and blink rate of participants in order to measure eye strain.
The second factor involves how easy a text is to understand mentally, not just visually. A number of characteristics of readable texts have been identified, most notably a few syllables per word, a few words per sentence, and a few uncommon words. These objective factors can be calculated by use of a number of readability tests, such as the Gunning Fog index or the Dale-Chall formula. Such tests often score readability based on the number of years of school that would be required to understand a text. For example, a text that scored 6.0 on the Gunning Fog index could probably be read and understood by someone who had completed six years of formal education.
Some studies can be used to measure either type of readability. For instance, rate of work studies measure how quickly participants are able to read and understand a text. Quantifying fatigue may also have both physical and mental components.
In addition to these measurable characteristics of readable writing, some researchers include subjective factors such as interest. For example, if a person finds a piece of writing interesting, he or she is more likely to retain the information in it. Although interest is difficult to quantify mathematically, studies have indicated that test participants do read more quickly when they rate the material as interesting.
Improving readability is often a matter of breaking down long, complicated sentences into shorter ones, and substituting simple words for unnecessarily complex ones. It is still important to vary sentence length and word choice to hold the reader's attention. Although research has been done to quantify what makes text readable, good writing remains as much an art as a science.