Read­ab­il­ity Index – what is it?

Whether blogger, marketers, or online editors, anyone who writes online texts must make sure they contain certain elements. Only then can they achieve a good ranking in the search engine results. In on page SEO, for example, keywords, headlines, links, and graphics are classic ranking factors. What SEO spe­cial­ists should not forget, however, and copy­writers need to take into account, is that good read­ab­il­ity is also important. Even if this is not (yet) of­fi­cially part of Google’s ranking factors, SEO analysts have found out: texts with good read­ab­il­ity generally rank better. Therefore, it is reas­on­able to assume that Google’s algorithm also registers a text’s read­ab­il­ity. And last but not least, good read­ab­il­ity is es­pe­cially important for website visitors and their user ex­per­i­ence.

But what exactly does the term 'read­ab­il­ity' mean? Read­ab­il­ity refers not only to ty­po­graphy and font size, but also to a text’s com­pre­hens­ib­il­ity.

Different math­em­at­ic­al methods have been developed to formally determine the read­ab­il­ity of a text. Two criteria that can be de­term­ined quickly using ap­pro­pri­ate analysis tools are, for example, the length of the sentence and the word length. These and other factors are related to each other by math­em­at­ic­al formulas and the so-called read­ab­il­ity index is cal­cu­lated from them. This read­ab­il­ity index does not replace human judgment, but it does provide an in­dic­a­tion as to whether the text meets the formal criteria for good read­ab­il­ity.

The read­ab­il­ity of texts is currently being discussed, mainly in the area of SEO. This aspect, however, has been important for a long time – as long as texts have been around – since people want them to be as easy to un­der­stand as possible. The most popular read­ab­il­ity index, the Flesch Reading Ease Index (in short: FRE Index), is older than the internet. It was developed by the author Rudolf Flesch, who re­searched the topic of usability in the 1940s and 1950s. Another model for eval­u­at­ing text com­pre­hens­ib­il­ity is the Hamburg Concept of Com­pre­hens­ib­il­ity. A Hamburg research group developed this in the 1960s and 1970s to simplify of­fi­cialese. The purpose of this sim­pli­fic­a­tion was to reproduce in­form­a­tion in such a way that all pop­u­la­tion groups could un­der­stand it equally.

There are many other models in addition to these two, meaning that the read­ab­il­ity index is not the same in every case. The in­di­vidu­al measuring methods create different criteria, are developed for different languages, and sometimes have very special target groups or text forms in their sights. When it comes to journ­al­ist­ic texts, however, the ob­jectiv­ity criteria must be weighted higher than it would be for a marketing text.

In this section, we present three read­ab­il­ity indices: the Flesch Reading Ease Index, the Flesch Kincaid Level, and the Fog Index. All three have proven them­selves in many areas. We present their formulas and criteria and give some sample analyses.

The Flesch Reading Ease Index, shortened to Flesch-Index, is a classic among the read­ab­il­ity formulas. It takes two key figures into account when cal­cu­lat­ing read­ab­il­ity:

1. The average sentence length (ASL)  
2. The average number of syllables per word (ASW)

This test was ori­gin­ally developed for the English language and the formulas differ slightly for different languages.

The higher the read­ab­il­ity index, the easier it is to un­der­stand the language of the text. The values are graded in a table according to the degree of com­pre­hens­ib­il­ity. This way, the table shows which groups can un­der­stand the text. For the higher levels, the read­ab­il­ity index refers to age groups: a text with a value of 90-100 is con­sidered very easy and un­der­stand­able for 11-year-old students. The lower values, on the other hand, are assigned to certain edu­ca­tion­al levels: texts with values between 0 and 30 can only be un­der­stood by academics. But even if you address this target group, easy read­ab­il­ity is still desirable.

The Flesch Kincaid Grade Level (abbr: FKGL) is another read­ab­il­ity index. The name already indicated its re­la­tion­ship with the Flesch Reading Ease Index: both were developed in the 1970s for the US Navy and were based on Rudolf Flesch’s research. They are also referred to as Flesch Kincaid Read­ab­il­ity tests. They are used by Microsoft Office Packet for read­ab­il­ity testing. In the 1990s, the US De­part­ment of Defense used the Flesch Kincaid index as a standard test for read­ab­il­ity.

Both tests evaluate the same core criteria (ASL and ASW), but they differ when it comes to weighting: the sentence length is weighted more strongly with FKGL than with FRE: In addition, the Flesch Kincaid Index differs more between age groups, es­pe­cially according to American grades. A value of 8.0, for example, means that the text in question is easy for eighth graders (Year 9) to un­der­stand, while a value of 7.0 means that even seventh graders (Year 8) can un­der­stand the text.

This test was ori­gin­ally developed for the English language. If you calculate the read­ab­il­ity index using the Flesch Kincaid formula for the first Harry Potter book in the series: Harry Potter and the Philo­soph­er’s Stone, you will get a score of 5.75, meaning this formula cal­cu­lates it a little lower than the Flesch Reading Ease Index at 81.32 (mentioned above) due to the different weighting factors. Here are some examples of average scores of famous people:

Stephen King – 6.1

J.R.R. Tolkien – 6.5

John Grisham – 7.8

Hilary Clinton – 8.1

Mitt Romney – 9.5

Susan Cain – 11.1

This test was also developed for the English language – the clas­si­fic­a­tion is based on American High School grades. It was developed in 1952 by busi­ness­man Robert Gunning, and is par­tic­u­larly popular in the man­age­ment sector. In this respect, it isn’t sur­pris­ing that it is fre­quently used to evaluate annual reports.

The Gunning Fox Index evaluates different key figures to the pre­vi­ously explained analysis metrics:

1. Number of words in a text (W)
2. Number of sentences in a text (S)
3. Number of 'complex words' with three or more syllables (D)

The read­ab­il­ity index is cal­cu­lated from these figures.

The values are graded in a table, ordered according to the education level needed to un­der­stand the text.

This text also sets out some guidelines that must be taken into account when accessing the text. You should test a text passage with only 100 words. In addition, some words will be excluded to determine the value D, e.g. proper nouns or word com­pos­i­tions and verbs that only have more than three syllables due to an ending being added.

Most texts on the internet are intended to provide quick and clear in­form­a­tion. Read­ab­il­ity is therefore essential. Only texts with a high degree of legib­il­ity are easy to un­der­stand.

Ex­per­i­ence from SEO experts shows that texts with a read­ab­il­ity of between 30 and 60 (FRE) are better. For this reason, it is assumed that read­ab­il­ity in some way also in­flu­ences the search engine ranking. It is therefore advisable to use a read­ab­il­ity index as part of search engine op­tim­isa­tion and to check the text content ac­cord­ingly.

Here’s a list of some online tests with which you can check your own content for legib­il­ity:

1. An online test that uses the Flesch Index can be found on the site, Readable.io.
2. The Flesch Kincaid Grade Level is used by the text veri­fic­a­tion tool, Per­ry­Mar­shall.
3. The Gunning Fog Index has its own website where you can test your text for free with the read­ab­il­ity index.

If the tests yield very high values, you should try to delete un­ne­ces­sary technical terms, foreign words, and ab­bre­vi­ations from the text.

If you want to ensure good legib­il­ity of your texts when writing them, you can use the Hamburg Com­pre­hens­ib­il­ity model as a guide. This model uses four quality criteria to evaluate texts: sim­pli­city, structure, concision, and stim­u­la­tion.

1. Sim­pli­city
   • Favour shorter sentences (ori­ent­a­tion value: approx. 9 to 13 words)
   • Favour shorter words
   • Include the most important in­form­a­tion at the start of the sentence
   • Express only one thought per sentence
      
2. Structure
   • Favour shorter para­graphs (ori­ent­a­tion value: up to 90 words)
   • Ap­pro­pri­ate length of chapters under the sub­head­ings (ori­ent­a­tion value: up to 300 words)
   • Include the most important in­form­a­tion at the beginning of the text
      
3. Concision
   • Use many verbs
   • Formulate actively
   • Use fig­ur­at­ive language
   • Work with trans­ition­al words (ori­ent­a­tion value: approx. 30 % of the sentences should contain trans­ition­al words)
      
4. Stim­u­la­tion
   • Include images and graphics to sup­ple­ment the text
   • In online marketing, stimulus and signal words such as 'great', 'easy', or 'cheap' can have a stim­u­lat­ing effect. For other kinds of texts, you should use these words carefully. 

When con­sid­er­ing the criteria of the Hamburg Com­pre­hens­ib­il­ity model, there are some things you should make sure to avoid when writing:

• Long sentences
• Technical terms and ab­bre­vi­ations
• Passive con­struc­tions
• Fillers
• Modal verbs (could, might, would, should)
• Nominal style

If you take these guidelines into account, your texts will be easier for readers to un­der­stand in the future and your website visitors will be more satisfied.