The concept was developed in the early 1970s at the University of Hamburg by the psychologists Tausch, Langer and Schulz von Thun. It starts from the idea that it is the nature of the wording that causes readers not to understand a text. The approach to solving the concept identifies four text characteristics that determine the degree of comprehensibility.
Simplicity: This characteristic refers to the choice of words and sentence structure. Content should be presented with short simple sentences and common words. Technical terms used should be explained, and readers should be able to imagine something under the content.
Structure: The internal order evaluates the consistency and meaningfulness of given information. The external structure evaluates the structural properties of a text, such as typographic markups, paragraphs, headings, and summaries. The inner order should be reflected in the outer structure.
Brevity/conciseness: This characteristic evaluates the length of a text in relation to the goal of information, whereby a concise, concentrated style contrasts with a lengthy one.
Stimulating additives: It is examined here how strongly the interest of the readers is aroused by stimulating additions. It should be noted, however, that many additions can make a text verbose.
In practical use, the four characteristics are arranged in an assessment window. Each of the characteristics can be assessed on a five-point scale, e.g. from -2 to +2. If a text is very strongly influenced by a characteristic, it is evaluated at +2. Not all features are equally important for optimal intelligibility. Very important are simplicity and structure/order, with the optimum at +2. For brevity/impressiveness, both extremes inhibit understanding. Likewise, many stimulating additives lead to prolixity. For both criteria, the optimum is between +1 and 0.