This article deals with the analyses of two parameters of instructional texts: a) the extent of the instructional text, b) the difficulty of the instructional text. Part One describes the most important methods worked out abroad (especially in the USSR, the GDR and Poland) for measuring the degree of difficulty of instructional texts. They are, on the one hand, statistical methods (based on the evaluation of the frequency of various language and content characteristics of the instructional texts), on the other hand, psychological methods (based on the appraising statements on the difficulty of instructional texts by various groups of judges). In each case an exposition is given of the experimental research verifying the didactic reality of results of the respective methods in the process of the acquisition of instructional material by pupils.
Part Two gives basic data on the difficulty of 15 instructional texts for different subjects in the 4th to 6th grades of the Czech elementary school. Use was made of the measure of difficulty of K. Nestler’s (1974) and J. Mistrik’s measure of comprehensibility of the text (1969). The analysis verified the hypothesis of the dependence of the difficulty of instructional texts on the pupils’ age, and of gradually increasing difficulty of instructional texts in accordance with the pupils’ age. This hypothesis was not confirmed: the analysis showed considerable disproportions in the degree of difficulty both between instructional texts of different subjects for the same grade and between instructional texts of the same subjects for different grades. For instance, some textbooks for a lower grade (History 5, Natural Science 5) were found to be more difficult than the textbooks of the same subjects for a higher grade. Sources of extreme difficulty are pointed out.
In Part Three it is pointed out that measures of difficulty can be used for prediction purpose, i. e. for the evaluation and correction of textbooks before their introduction into schools.