David Kusserow



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WDF*IDF is a mathematical formula that you use to determine a value that puts the occurrence of a word in a text in relation to all the occurrences available to a search engine.


The cryptic abbreviation WDF*IDF stands for “Within Document Frequency” in relation to “Inverse Document Frequency”. WDF is still understandable and measures the frequency of words in a document and categorizes them according to a weighting . The concept IDF refers to the evaluation of a word by the index of a search engine.

This search engine indexing shows that this formula is of vital importance for the search engine optimization (SEO) of your content. With the WDF*IDF formula you calculate the ratio of certain words – for example your keywords – in your document in relation to all documents that could possibly be available to the search engine.

The formula incorporates term frequency – how often your keyword appears in Google hits – and is important for your OnPage optimization. If you use your WDF*IDF analysis cleverly, you can increase the authority of your website . This will help you achieve a higher position on the Search Engine Result Page (SERP).

Because it can be difficult to calculate especially the IDF factor in a WDF*IDF analysis, there are several WDF*IDF tools online that you can use for your SEO.

Within Document Frequency

This value does not only indicate how often a word occurs in your text. Unlike keyword density – the percentage frequency of your keyword relative to the total number of words in the text – WDF is designed to prevent keyword spamming from working. Therefore the WDF value is calculated by the formula

WDFi= log2(Freqi,j+ 1) / log2(L).

The logarithm does not only consider the frequency of one word, but the frequency of all words.


i = word
j = document
L = total number of words in document j
Freq(i,j) = frequency of the word i in the document j

Inverse Document Frequency

The IDF factor puts the WDF factor of your text into a broad context. In order to apply the formula IDFt= log (1 + ND/ ft), you must first determine the relevance of the keywords with a keyword analysis. For a thorough analysis, you need to perform WDF*IDF analysis for every important word in your content. Only now can you make reliable statements about the weighting of your keywords with regard to their appearance on the Internet.


N_D = Number of documents

f_t = Number of documents containing the term t

WDF*IDF benefit for search engine optimization

With the WDF*IDF formula you can see how much the texts of your website differ from those of your competitors. The result gives you a good basis on which you can align your OnPage optimization. The goal should be for you to write as unique copy as possible that will be presented higher up on the SERP by the search engine through clever keyword selection. Although a WDF*IDF analysis is much more complex than a keyword analysis, it is your tool of choice to write search engine optimized text. With the results in mind, you can formulate your topic much more precisely for the crawlers. A good tool for such an optimization would be, for example, a semantic analysis.

Difficulties with WDF*IDF

The WDF*IDF is only a mathematical basis for a more precise keyword analysis, which helps you to create your content as individual as possible. It’s also worth thinking about which signal words are relevant beyond the keywords you’ve examined. Often, these words can only reveal what a user is really looking for. You should also not lose sight of the many other means of search engine optimization.

The mathematical basis of WDF*IDF shows at the same time the limits of WDF*IDF analyses. The tools only weight how often a word occurs. Where it appears or whether the context is right is of no consequence. One of the most important aspects of good SEO work is that you target your content not to crawlers, but to your human users. You can achieve a long dwell time and a low bounce rate through informative, appealing and easy to read texts. The behavior of users on a web page is valued more by algorithms today than in the past The authority of your page on the SERP strongly influences the quality of your content.

Also in online shops the use of the WDF*IDF formula is limited, because the pages often present only little text.


A WDF*IDF analysis is more accurate than a keyword analysis in finding the right keywords. It is thus a good way to check if you are writing unique texts that appear high up on the Search Engine Result Page. But the analysis makes no statement about the quality of your texts. Just the ratio of words in your text compared to all possible search engine hits does not show whether your content is attractive to your users.

As search engines today are increasingly weighting user behavior key performance indicators (KPI) such as dwell time and bounce rate in their algorithms, the usability of your site is becoming more and more important. Therefore, you should find a way to write informative, engaging, and enjoyable copy. With a little skill you can still get good values using the WDF*IDF formula.


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