How a classroom should be selected for observation and how the observer should behave while observing? There are many psycho-social aspects of a classroom. how to deal with these while observing the class?
Birdwell, T., Roman, T. A., Hammersmith, L., & Jerolimov, D. (2016). Active learning classroom observation tool: A practical tool for classroom observation and instructor reflection in active learning classrooms. Journal on Centers for Teaching and Learning, 8.
Lasagabaster, D., & Sierra, J. M. (2011). Classroom observation: desirable conditions established by teachers. European Journal of Teacher Education, 34(4), 449-463.
Most classroom observations often include both the instructor and class preparing for a “personal visit” observation. They prepare to showcase their best behavior and an impactful lesson. Even if the visit is unplanned, both instructors and students change their normative behaviors. I find, multiple visits with the same observer building rapport and establishing neutrality works. Multiple visits allow for a more holistic assessment and accounts for good, bad, and variable days - i.e., accommodates the variability. If you’re going to assess, do it right. Take the time. Argue for the support. It’s worth it. The known personality of the observer is also a factor. There are other methods, but all need to consider and follow ethical protocol. As for selection/sampling, it depends on what you are addressing (a research question, class/instructor performance, problems, group dynamics...?). Also, many people tend to simplify - with a few Likert scale questions and notes. Not recommended. Having a broad spectrum of Likert scale questions will allow cross-checking and enhance nuanced richness (doesn’t take long whether it’s 20 or 70 entries if the instrument is constructed adequately). Also, have a fair balance of quantitative and qualitative assessments. Get feedback from several people in the classroom. Do the deep data approach. Make it a positive feedback exercise for all stakeholders.