The view on the professional role of teachers is subject to change, as is the view on how teachers need to develop professionally. In the past, teachers were seen as autonomous professionals who worked and professionalized isolated in their own classroom. In the last two decades, this view proved no longer sufficient and teachers were increasingly expected to professionalize in collaboration (Hargreaves, 2000; Verloop, 2003).
One of the main developments in professional development of teachers is peer review and peer feedback (Thurlings, 2012). In a review study, Coe, Aloisi, Higgings, and Major (2014) discuss a common form of peer review and – feedback: classroom observation followed by a formative, collegial consultation. Although they underpin the effectiveness of this approach, they highlight that the literature emphasizes to involve not only peers (from now on called ‘internal observers’) in classroom observations, but also other observers, such as external observers from outside the school. Several reasons can be given why different observers might be needed.
First, there are some difficulties with observations in general that might affect observations from internal and external observers in a different way. From a methodological viewpoint, a challenge in observation lies in the quality of the observer’s inference: the observer needs to translate the observed to the construct of the observation instrument the most objectively as possible (Kerlinger & Lee, 2000). Differences between observations done by internal and external observers might occur due to the fact that internal observers already know the teacher, the class and the school.
Second, issues regarding the relationship between teacher and observer can potentially influence the observation. Coe et al. (2014) discuss ‘political issues’ that play a role in classroom observation, such as trust and authority. Hobson and McIntyre (2013) report that teachers seem more unwilling to welcome an internal observer to their classroom, because of fear to the opinion of colleagues. An internal observer might be more sensitive to these kind of feelings of an observed teacher. If the observation and consultation disturbs the relationship between the observer and teacher, the impact in the case of peer observation is greater on both observer and observed teacher and the risks are higher, since afterwards they still see each other on a regular basis.
In addition, research by Hargreaves (2004) gives us an interesting insight in collegial interactions among teachers. His research shows that collegial interactions have the tendency to focus on supporting each other instead of discussing each other’s teaching practice. Colleagues tend to avoid disagreement. If this is the case, development may be counteracted or even ineffective teaching behavior is perpetuated.
Studies on the differences between the valuation of a lesson by an internal or external observer are rare and there is no empirical evidence yet of any potential differences. Given the aforementioned general challenges of classroom observation and the issues regarding the relationship and interactions between observer and observed teacher, our hypothesis is that internal observers are less critical than external observers when performing a classroom observation.
This leads to our research question: are internal observers from the own school less critical when observing a lesson than external observers from outside the school?

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