Limited numbers of study (Aydın & Boz, 2012; Pine, Messer, & John, 2001) have attempted to directly study teachers’ knowledge about assessment (Abell, 2007). Teachers’ knowledge about assessment contains two categories: knowledge about the science learning that is important to be assessed in a specific unit and knowledge about the assessment methods (Magnusson, Krajcik, & Borko (1999). In the first category, the teachers should be aware of the aspects of scientific literacy to be able to assess students’ conceptual understanding, interdisciplinary ideas, and scientific reasoning and investigation in a specific unit (Champagne, 1989). The second category includes the knowledge about assessment tools that can be used to assess the important dimensions of students’ science learning along with knowledge about the advantages and disadvantages of using these tools in a specific unit. The teachers should be aware of which assessment tools or procedures to be used in order to assess students’ understanding of science concepts, and scientific reasoning and investigation (Magnusson et al. 1999). The research on science teachers’ usage of assessment tools showed that the science teachers mostly use teacher-generated tests to assess the students’ understanding of the scientific concepts (Doran, Lawrenz, & Helgeson, 1994). The student-generated science products, such as, journals, portfolios, laboratory reports, or multimedia products on the other hand, yield important advantages wherever they are used in the teaching process (Kulm & Malcom, 1991).

Although there are many studies investigated in-service teachers’  PCK (Balta& Eryilmaz, 2011; Beyer & Davis, 2011;  Coruhlu & Cepni, 2010; Hanuscin, Lee, & Akerson, 2010; Karakulak & Tekkaya, 2010; Nezvalová, 2011; Nuangchalerm, 2011; Syh-Jong & Hsiu-Chuan, 2009), there is a need for studies in the literature investigating the inservice teachers’ knowledge about assessment strategies in classes with different academic success levels in order to shed more light on the nature of in-service teachers’ PCK. The assessment is one of the dimensions of PCK that the teachers experience  the biggest problem about (Aydin & Cakiroglu, 2010). Therefore, the current study will focus on an experienced science teacher’s knowledge about assessment strategies in two classes with different academic success levels based on Magnusson et al. (1999)’s PCK model. Following research questions will guide the present study;

1. What is the nature of a science teacher’s knowledge about assessment?

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