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We can trace the origin of readability formulas during the late 19th Century in the United States. The schools in the US did not routinely grade students until 1847 when the first graded school opened in Boston with a series of books prepared for each grade. Junior high school science teachers wanted to teach scientific facts and methods in plain English, rather than teaching complicated science vocabulary. As a result, teachers, librarians, and scholars developed primitive readability formulas to determine what seemed readable to their students.

No one had ever entertained the thought of grading adults. The US Military took the first step in grading adults in 1917. Chicago took the clue from the military and started grading civilians in 1937. Earlier studies had revealed that general adult readers in the United States had limited reading ability.

The breakthrough of readability formulas was the publication of The Teacher's Word Book by Edward Thorndike in 1921. In his book, Thorndike researched how often general literature used difficult words. For the first time, a notable scholar suggested a way to measure difficult words through mathematical formulas.

Early researchers studied surface characteristics of written texts to determine the extent readers could comprehend texts. Then they compared the data with certain predetermined standards; one such standard was tabulating the average grade level of students who could correctly answer a certain percentage of questions from the text. The characteristics with the most accurate standards were judged as indices of readability. These characteristics were worked upon and developed into readability formulas.

Thorndike's book was followed by another landmark work by George Kingsley Zipf in 1949. Zipf came up with Human Behavior and The Principle of Least Effort, in which he declared a mathematical relationship between the hard and easy words, called Zipf's Curve.

Many researchers have contributed to developing readability formulas. Rudolf Flesch is one of the better known developers of a readability formula called Flesch-Kincaid Grade Level. His formula uses the number of syllables per 100 words and the average number of words per sentence. Flesch said that writers can make their texts easier to read by using shorter words and shorter sentences.

In 1963 George R. Klare published his book, The Measurement of Readability, in which he reviewed the efforts to improve comprehensible language by revising the texts to lower readability scores. He also suggested using readability formulas as an aid to increase effectiveness of writing and speaking. Klare published over 80 papers and studies on readability in professional and scholarly journals. He died in 2006.

Some of the other significant contributors of readability formulas include Edgar Dale, Jeanne Chall, Robert Gunning, Ed Fry, Tom Trabasso, and J. Peter Kinkaid, etc.

Presently, there are over 200 readability formulas with varying degrees of accuracy and success rate. Many scholars debate about which readability formula is foolproof. All formulas have some significance in improving text readability.

Disadvantages of Using Readability Formulas

1. "Readability" is different from "understand-ability." Unfortunately, readability formulas are not much help if you want to know if the target audience will understand the text.

2. Readability formulas also cannot measure the complexity of a word or phrase to pinpoint where you need to correct it.

3. The admirers of literary geniuses largely see readability formulas as an affront to their work. Most great literary works fail to pass the readability tests, but this doesn't mean that those works are inferior in quality. The critics view readability formulas as over-simplification and a critique of creative writing.

4. Due to many readability formulas, there is an increasing chance of getting wide variations in results of a same text.

5. Readability formulas cannot measure everything that contributes to how readable a book is for a student, any more than a reading test can measure a student's reading behavior.

6. Readability formulas can't measure the context, prior knowledge, interest level, difficulty of concepts, or coherence of text.

7. Readability formulas apply mathematics to literature. This idea, itself, is not favored by language scholars and researchers.

Advantages of Using Readability Formulas

1. By definition, readability formulas measure the grade-level readers must have to read a given text. The results from using readability formulas provide the writer of the text with much needed information to reach his target audience.

2. Readability formulas do not require the (targeted) readers to first go through the text to decide if the text is too hard or too easy to read. By using readability formulas, you can know ahead of time if your readers can understand your material. This saves you time and money.

3. Readability formulas are text-based formulas; many researchers and writers find them easy-to-use.

4. Readability formulas help the text-creators convert the document into plain language if the readability levels are low (which is the case with the reading levels of many American people), or too high (which is normally the case with higher-studies' students, researchers and professionals).

5. Using readability formulas to perfect a document can help readers to increase their retention, comprehension, and speed of reading; this, in turn, smoothens out the work-schedule of your readers.

6. A readable text always attracts a larger reader-base.