The SMOG Index is an influential and popular writing tool. One that helps content writers score their work for readability and clarity of message.  But, where did it come from, and how can you best use it?

What is a SMOG Index readability score?

You may be asking, “Why SMOG?” Well, SMOG stands for ‘Simple Measure of Gobbledygook’.

It is a readability framework which can be used to analyze how readable text is. It assesses how many years of education the average person needs to have to understand a piece.

It is most accurate for texts which are comprised of 30 sentences or more, as this was the length of text sampled in the creation of the formula.

SMOG, among other formulas, is used in ReadablePro. Its unique algorithms create an overall readability score of a text.

Where did the test come from?

The SMOG Index was created by G. Harry McLaughlin, a psycholinguistics graduate and a research and clinical psychologist.

McLaughlin obtained his PhD at University College, London. This led him to a teaching role at The City University in London for six years. He then taught in Toronto and Syracuse.

He was very interested in finding a simple formula which takes into account polysyllabic words. McLaughlin made it his mission to create the most reliable readability calculation. He called his own formula “laughably simple”.

Indeed, when it was first published in 1969, it was doubted at first for its simplicity. However, it’s been proven to be an excellent measure of readability and is still used today.

McLaughlin also named it SMOG as a nod to Robert Gunning’s formula. Gunning created the FOG Index, also in the 1960s.

Read our article on the Gunning Fog framework for more information on the FOG Index.

SMOG is also a homage to McLaughlin’s hometown. London suffered a Great Smog due to severe air pollution in the early 1950s. The use of terms like ‘fog’ and ‘smog’ in readability studies highlights the need to promote clarity of writing.

London SMOG

How does the test work?

The formula estimates the years of education the average person needs to understand any piece of writing. This is known as the SMOG Grade.

SMOG Index readability formula

McLaughlin suggested calculating this by using a piece which is 30 sentences or longer and doing the following:

  1. Counting ten sentences near the beginning of the text, 10 in the middle and ten near the end, totaling 30 sentences
  2. Counting every word with three or more syllables
  3. Square-rooting the number and rounding it to the nearest 10
  4. Adding three to this figure

The final figure indicates what reading grade a person must have reached to comprehend the text fully.

If the formula sounds remarkably simple, that’s because it is. A statistician who looked at an earlier version of the paper thought it was a “put on”. He couldn’t believe this method was so simple.

Nonetheless, McLaughlin’s claims of validity have been backed up by research, particularly in healthcare literature.

The SMOG Index was created in the pre-computer age. Going beyond the sampled length of 30 sentences would certainly be laborious to calculate manually.

ReadablePro analyses the text as a whole - all you have to do is paste it in. It uses the SMOG Index along with a variety of other tried-and-tested formulas to create a unique and easy-to-read score.

Test your text with ReadablePro today for a free readabilty report.

When is the SMOG Index most useful?

Although SMOG is widely used, healthcare is the sector it is mostly used in. Its exacting framework measures the reading grade required for complete comprehension. Medical use of the formula has been helped along by research into different measures of readability and their usefulness in healthcare.

For example, one case study assessing the readability of online Parkinson’s disease information called SMOG the ‘gold standard’ in measuring this.

SMOG index, often used in healthcare

Their research on online literature was prompted by an increase in patients using the internet to find health information.

Of course, patients are less literate in healthcare-related terms than medical professionals and struggle to comprehend healthcare literature as a result.

Their focus was on consumer-oriented healthcare information and the consumer’s best interests.

They used a couple of different formulas but found that for this particular healthcare purpose, ‘Simple Measure of Gobbledygook should be the preferred measure of webpage readability’.

In this particular case study, with at least part of the Parkinson’s Disease population regularly using the internet, the conclusions of the paper are concerning. They are also a testament to the necessity of exacting formulas such as SMOG.

They found that the majority of consumers cannot understand health information provided online. They also note that readability in printed literature is a long-standing problem.

In an age of using WebMD in an attempt to understand our illnesses before visiting the doctor, the confusing terms in online health literature are a cause for concern.

Clarity of message is key. This is why efforts made by McLaughlin and now by ReadablePro are not just a necessity for consumers in general, but also for public health.

We are now seeing this issue crop up increasingly in the news. In the UK, there has been an influx of reports concerning a drive for doctors to write to patients in ‘plain English’. This means avoiding the overuse of convoluted sentences and medical jargon.

This development comes along with a major potential change in the nature of medical letter-writing. Traditionally, patients are copied into correspondence between a doctor and a patient’s GP.

The Academy of Medical Royal Colleges is now urging doctors to write directly to patients in language they can understand. This would reduce the number of appointments patients make with their GPs that are made just to have letters explained in plain terms.

For some examples of terms which doctors will likely need to simplify, we have previously talked about new readability practices being adopted in healthcare.

So, although McLaughlin called SMOG a “laughably simple” tool, it’s had an impressive impact! Its main aim is to encourage clarity of message, which we at believe is important in any sector.

Although this framework has lent itself especially to healthcare, combined with ReadablePro’s bespoke collection of algorithms it can help you identify linguistic smog, whatever your message is.

Laura Kelly

Laura is a content writer and customer success champion at Readable. She's a Literature MA graduate who loves poetry, a good coffee and 35mm photography.