Although it was suggested earlier that interest had to be a key criterion in selecting texts, it is clearly counter-productive to choose texts with too much difficult vocabulary. Students need to be confronted with challenges and left to guess or work out the meaning of some new words from context, but they must be able to understand most of the rest of the text in order to do this (see Module 7, section 7.5.2.2). As a rule of thumb, six to eight new words per hundred is really the maximum we should be confronting learners with; otherwise we are asking them to cope with too much new material at once. Even if thorough preparation is first carried out with a dictionary, this may not help students significantly - many students find that even after intensive dictionary work on a text with a high density of new lexis, they still struggle to grasp the overall meaning. This is because the difficulty of a text increases exponentially in relation to the number of new words used: ie twice as many new words do not double the level of difficulty, but more likely treble it.

Thus the 'interest' criterion has to be interpreted in the light of the likely difficulty of the text for the target group. No matter how imaginative the exploitation of the text, if it is too difficult the result will be a slow, word-by-word and thoroughly demoralizing struggle.