In the time when learning is getting increasingly supported by ICT and new media, the role of printed school textbooks in implementing the curricula is changing. In many European countries the textbook is no longer the only and perhaps not even the main source of information. But it is still an important source of knowledge in a more specific sense of this word. Its main role is in enabling the students to gradually appropriate the main conceptual networks used in an area of study, and to control the coherence of interpretation of data obtained from other sources. Therefore the educational publishers should pay far more attention to the qualitative than to quantitative aspect of textbook production. Most of the existing textbooks can be improved. One way of demonstrating this is in carrying out experimental studies. Several studies carried out in various European countries have already shown that the learning effectiveness of samples of textbook material can be significantly improved if they undergo a thorough, well considered revision. Most of these studies manipulated samples in just one dimension, e.g. in the dimension of readability, abstractness or coherence. It is due to this one-dimensional approach that researchers could control and describe the changes introduced into the revised version of samples. Some of the studies manipulated samples in several dimensions but this made the control of the changes more difficult and there was little report on them. In our quasi-experimental study we carried out several effective multi-dimensional revisions of textbook passages while controlling for the exact changes introduced into their structure. A selection was made of four passages from four different geography textbooks used in Slovenian schools by 9th graders. Each of the passages consisted of about 300 words, their cognitive contents being complete in themselves. The passages underwent a thorough revision conducted by a group of experts in psycholinguistics, didactics, developmental psychology, discourse analysis and geography. In the revised versions the core content and the size of the passages were preserved. Four groups of students read the passages. Immediately after that their comprehension of passages was measured by multiple-choice tests. The main research questions were: How much does a thorough and well controlled multi-dimensional revision of passages from existing textbooks contribute to an increase of students’ comprehension of basic ideas and concepts? How much does it affect the comprehension in two groups of students, in the groups of high- and low-achievers.

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