STEM education has gained worldwide importance in the last decade (Dare et al., 2018), and attempts to integrate STEM in K-12 education have been increasing (Tanenbaum, 2016). Definition of STEM education is still messy; however, some common points could be inferred from STEM definitions. Examples of the commonalities contain: using authentic and engaging contexts (Breiner et al., 2012), utilizing engineering design process (Guzey et al., 2016), promoting teamwork and communication (Moore et al., 2015), improvement of 21^st century skills (Honey et al., 2014), explicitly integrate concepts and practices of different disciplines (English, 2016), supporting evidence-based decision making and learning from failure (Wendell & Rogers, 2013), and using science and mathematics disciplines to solve problems (Rinke et al., 2016). Based on these commonalities, the basic characteristics of STEM education in our study were: science is the lead discipline, concepts and practices from other disciplines are embodied in science and using real-world contexts to solve problems through the engineering design process.   

Although there has been increasing emphasis on STEM education, how pre-service teachers should be trained to implement STEM education effectively in their classrooms remains unclear (Rinke et al., 2016). Teachers need a different type of knowledge to implement STEM lesson plans in their classrooms, and pedagogical content knowledge (PCK) is considered as a useful framework at this point (Vossen et al., 2019). The importance of PCK for STEM is described as “how to meld engineering and their disciplines effectively in classroom instruction. Teachers must have the PCK to help them expand beyond science or mathematics to include defining and delimiting engineering problems, designing solutions, and optimizing designs” (Lau & Multani, 2018, p. 196). Some studies utilizing PCK framework for STEM proposed new components and definitions for PCK (Allen et al., 2016; Saxton et al., 2014). Among PCK models existed in the literature, Magnusson et al. (1999) PCK model was chosen in the present study. According to this model, there are five components of PCK: orientations to teaching science, knowledge of instructional strategies, curriculum, student learning, and assessment. Four components, except orientation to teaching science, were included and modified as “PCK for STEM” in the current study. Orientation to teaching science component was excluded since it is ill-structured (Friedrichsen, et al., 2011) and less-content specific (Henze & Barendsen, 2019). Research on PCK for STEM is limited (Hynes, 2016; Lau & Multani, 2018; Aydın et al., 2020), and literature calls for longitudinal studies to improve pre-service teachers PCK for STEM (Vossen, et al., 2019). Moreover, experience and reflection are the two important sources for developing PCK (Juhler, 2016; Nilsson, 2008), and lesson study was utilized as a professional development program to improve pre-service science teachers' PCK for STEM. Lesson study originally emerged from Japanese classrooms (Lewis, 2002); however, used frequently in the European context (Bjuland & Mosvold, 2015; Skott & Møller, 2020). Lesson study is composed of iterative cycles and provides an opportunity for reflecting on instruction (Dudley, 2007). The phases of lesson study used in this study were as follows: (1) identifying goals of the lesson, (2) collaboratively planning the lesson, (3) implementing and observing the lesson in a real classroom environment, (4) discussing and revising lesson as a group, (5) re-taught the lesson by another teacher from the group to another classroom and (6)re-reflecting on a revised lesson. The main research question guided the current study was: How do pre-service science teachers' PCK for STEM develop in the context of lesson study? There were sub-research questions parallel with Magnusson et al.'s (1999) PCK framework. This study is expected to be significant for making PCK for STEM development explicit through lesson study. 

The present study was qualitative, and multiple-case study was used (Creswell, 2013). Four pre-service science teachers participated, and each constituted one case in this study. Participants were in their senior year, and pseudonyms were used for confidentiality. Participants were asked to prepare four STEM lesson plans collaboratively; however, only the results of the first lesson study cycle were presented in the current study. The data were collected by means of several instruments to capture participants' PCK for STEM. Content Representation (CoRe) was utilized as a lesson planning tool. It was originally developed by Loughran et al. (2004), but a revised version of CoRe for pre-service teachers was used in this study (Aydın et al., 2013). The CoRe includes big ideas related to the topic to be taught in the horizontal axis and prompts about the instructional decisions in the vertical axis, such as students’ misconceptions, teaching procedure etc. Participants designed individual CoRe’s based on STEM education before participating lesson study cycle. After introducing the lesson study, they were asked to prepare CoRe collaboratively by considering the big ideas of the lesson in the planning phase. Solar Eclipse and designing Solar eclipse viewer as an engineering design challenge was determined as big idea of the lesson. The planning phase was completed in six hours, and these meetings were video recorded. After the planning phase, participants were interviewed individually before teaching. Then, Deniz taught the lesson to the 6th grade in three lesson hours, and other participants and researcher observed the lesson and filled out the observation protocol. Interview questions and observation protocol were prepared based on a modified version of the Magnusson et al. (1999) model and piloted before the main study. The researcher conducted post-teaching interviews to obtain participants’ observations and suggestions to revise the lesson plan. Later, the group met for the revision phase and discussed the strengths and weaknesses of lesson plan. They revised it, and Ada re-taught the revised lesson plan to another 6th grade classroom. After implementation, the group reflected on the revised lesson. Priori code list was prepared based on a modified version of the PCK model. New codes emerged after the first round of coding (i.e., strategies to implement the engineering design process), and then the code list was finalized. Codes were used to form categories (Patton, 2002). Categories were labeled as PCK-A and PCK-B based on the level of PCK for STEM development.

The present study utilized both group data and individual data since the PCK of each case was unique. Participants did not demonstrate the basic features of STEM education in their individual CoRe’s. All participants showed noticeable improvements in their PCK for STEM, but levels of development differed after completing the first lesson study. To illustrate, Ada had a more coherent PCK for integrating STEM education (PCK-A). As an example, she suggested to use strategies specific to the engineering design process in the planning phase, such as adding a decision table that included positive and negative aspects of students’ designs for the re-designing part. She used representations specific to the engineering design challenge and provided daily life examples to make students identify criteria and limitations in the engineering design challenge during the teaching phase. On the other hand, Deniz experienced some difficulties in terms of knowledge of instructional strategies. He offered a close-ended activity that ended up with one solution to solve engineering challenges in the planning phase and narrowed down possible solutions for designing a solar eclipse viewer to just two alternatives during teaching. This situation demonstrated that he was able to implement basic features of STEM education to some extent and had less coherent PCK (PCK-B). These findings were in parallel with Aydın et al.’s (2020) and Vossen et al.’s (2019) studies in which long-term exposure to professional development programs supports pre-service teachers' development of PCK to some degree. Moreover, Deniz and Ada implemented the same lesson plan; however, their lessons were different in some aspects, demonstrating that the development of PCK is unique to each teacher (Berry et al., 2008). Finally, findings suggested that collaboration, observation, and reflection were the three features of lesson study that provide a fruitful way to enhance pre-service teachers’ PCK for STEM.

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