Theoretical and empirical framework of instructional quality lies in various quality models designed for different levels/contexts of education. The individual models describe various aspects that influence the quality of instruction and those processes which are influenced by these aspects. These aspects include characteristics of the learner, the teacher, the class, the school and the wider educational context (educational system, curricular documents, the pupils’ family background, subjects etc.). The processes influencing the quality of instruction often include the processes of instruction and learning, mediation, testing and evaluation of learners.
As Helmke (2007, p. 41) points out, quality of instruction may be seen from two different perspectives: a) from the point of view of such teacher competences that are necessary for the realization of quality instruction; b) from the point of view of such aspects which are relevant to the evaluation of the quality of instruction. The author further describes various aspects which may affect the quality of instruction.
Helmke (2007, p. 43) defines quality of instruction as principles and characteristics essential for successful instruction. It relates to a superior universal principle of balance of clarity, adequacy of learners variation, subjects and situations, teaching methods, sensitive heterogeneity and individualisation handling and motivation. The secondary components of quality of instruction involve efficient classroom management, the quantity of instruction and the often-neglected quality of teaching materials. The attention is usually paid to various aspects of the quality of instruction: pacing (dynamics of adapting difficultness and tempo to a particular learning situation and conditions, etc.); clarity (acoustic comprehensibility – articulation, preciseness and language correctness, structuring of the content, etc.); variability of teaching methods and forms; individualisation; motivation; efficiency of classroom management; quantity of instruction (time allocated to instruction, time allocated to learning, etc.); learners’ cognitive activation; perception and interpretation of classroom processes (learners‘ reflective feed-back on the classroom processes with regard to previous knowledge, motivation and individual needs, etc.).
In this particular analysis we aim at clarity of instruction as an aspect of instructional quality. Clarity of instruction is operationalised as the realisation of structuring of the content as well as acoustic and language preciseness. Lessons of physics, geography, English as a second language and physical education were analysed; similarities and differences in the realisation of clarity of instruction were observed which helped to develop a comparative picture of the concept.

Helmke, A. (2007). Unterrichtsqualität. Seelze: Kallmayer. Najvar, P., Janík, T., Janíková, M., Hübelová, D., & Najvarová, V. (2009). CPV video study: Comparative perspectives on teaching in different school subjects. In T. Janík & T. Seidel (Eds.), The power of video studies in investigating teaching and learning in the classroom (pp. 103-119). Münster: Waxmann.