Many observation instruments are in use to make the skills of teachers visible. These tools are used for assessment, for guidance and coaching, and for policy-oriented research into the quality of education. Depending on the purpose of use of an observation instrument, we not only need more observations about the same teacher, but the observation instrument must also meet higher psychometric requirements. Observation instruments only used to assess sample characteristics, such as the mean and dispersion, require less stringent psychometric requirements than observation instruments that are used to assess individuals. For assessing sample characteristics, it is also not necessary to do more than one observation with each respondent. Observation instruments used for individual assessments that lead to high stake decisions should meet the highest psychometric requirements possible. We can slightly mitigate the psychometric norms attached to an observation tool that is only used for guidance and coaching on the condition that the observed teacher explicitly informed that the observed lesson was representative and that this lesson offered sufficient opportunities to demonstrate all the skills the teacher has. Nevertheless, there are also additional requirements that must be met by observation instruments that are used for guidance and coaching. For good guidance and coaching, it is usually not very useful to tell an observed teacher only what went right or wrong. Teachers need concrete instructions to be able to improve. Many things that have not gone very well are often (and sometimes far) out of the reach of the teacher being observed. Coaching skills that are beyond the reach of the observed person will lead to disappointment rather than to the desired effect. The important thing in good guidance and coaching is to ensure that the observed teacher is going to take that very step, that is within his reach, but that he has not just set. Then, of course continue with the next steps, leading to incremental progress. For this, we need to have an insight into the successive difficulty of the different skills of teachers. In the past, we gained some experience with the use of the Rasch model to gain an insight into the successive level of difficulty in the actions of Dutch teachers working in elementary education. These studies are all done with the International Comparative Analysis of Learning and Teaching (ICALT) observation instrument. In this chapter, we are trying to make a next step by using the Rasch model for detecting the zone of proximal development of the observed teachers. Another new element in this study is the following: Until now, the ICALT observation instrument has been used mainly in (the culture of) European schools. In this chapter, we focus on Asian secondary education, as it takes shape in South Korea.
