According to Assaraf and Orion (2005), last 20 years called as “Science for All” age in terms of science education. The important point of Science for All era is that the goal of education changes to educate our future citizens rather than scientists. In the 21st century, students’ worlds are motivated by complex systems that are dynamic and adaptable (Jacobson & Wilensky, 2006). According to Sabelli (2006), understanding complex systems important since it plays a significant role in individuals’ interactions with the world. They provide a context that integrates different scientific domains, and they are essential aspects of being a scientifically literate person, so they are a fundamental focus for science education (Assaraf & Orion, 2009; Next Generation Science Standards [NGSS], 2013). NGSS highlights “crosscutting concepts” as one of the basic aspects of scientific learning containing topics on system models, patterns, and stability and change. Moreover, complex systems are characterized by multilevel organizations with interconnections and invisible dynamic processes (Ferrari & Chi, 1998; Hmelo-Silver & Azevedo, 2006; Wilensky & Resnick, 1999). These features make complex systems difficult to understand.

Complex systems have micro and macro levels. So, one should understand these levels in order to understand the whole system. For example, unless a student understands the relationship among genes, individuals, population, and communities, s/he cannot learn the ecological system. Here, while genes represent micro level, community represents macro level, and ecological system is an example for complex systems (Hmelo-Silver, Marathe, & Liu, 2007). The human body also consists of micro and macro-level components, and it is given in the curriculum, so, students are not only expected to make connections between levels and also to understand human body as a system since it is in the curriculum (Tripto, Assaraf & Amit,2013). Since the human body has subsystems and there is a relationship among and between the systems, it is an example of a complex system.

Throughout the time, researchers state many different conceptual representations that can be used so that investigate understanding of complex systems. One of these frameworks is systems thinking. Senge (1990, p.69) defined systems thinking as “a framework for seeing interrelationships rather than things, for seeing patterns of change rather than static snapshots.” Teaching systems thinking skills within the context of science education is essential because many phenomena around us are examples of complex systems and understanding of their function requires systemic thinking (Evagorou, Korfiatis, Nicolau & Constantinou, 2009). Assaraf and Orion (2005) classify systems thinking skills in eight hierarchical characteristics according to difficulty for acquiring each skill. This is the Systems Thinking Hierarchical Model (STH) model of Assaraf and Orion, and STH model’s eight characteristics are gathered under the three sequential levels, which are analyzing, synthesizing, and implementation. Analyzing level includes identifying the components and processes of a system; synthesizing level involves identifying simple relationships among a system’s components, identifying dynamic relationships within the system, organizing relationships and interactions between the system components, and identifying cycles within a system and the last level implementation consists of identifying hidden parts of the system, making generalizations about a system, and thinking temporally.

The purpose of this study is to investigate middle school students’ systems thinking skill level regarding human body system in the framework of systems thinking hierarchy developed by Assaraf & Orion (2005). More specifically, the research question is investigated:

- What is the systems thinking level of Grade 7^th students regarding human body system?

- To what extent students understand human body system?

• How do students relate parts of the system to each other?
• How do students link parts of the system to each other?

Case study design is used in this qualitative study. Merriam and Tisdell (2015) argue that a case study is an in-depth description and analysis of a bounded system. Multiple case studies are suitable for the explanation of current study. Although each of the students will meet the study’s criteria and share some commonalities with each other, they will differ in their characteristics, their content knowledge and their level of systems thinking ability. Therefore, each of the participants will be separate case study in this multiple case study and each of the cases will be examples of phenomena. In this study, nonprobability sampling is used because in qualitative research there is no aim to generalize in a statistical frame and purposeful sampling is selected because it is relying on the things that researcher aims to discover (Merriam & Tisdell, 2015). The research population will approximately include 5 7th grade science students from one pubic school in Ankara, the capital city of Turkey. In this study, convenience sampling which is one type of the purposive sampling will be used since acceptance to participate study and location are important. Accordingly, in this study, researcher will participate in the all data collection process and will conduct all interviews in person. Interview process will take approximately one hour for each participant. Interviews were conducted in a quiet and comfortable place in students’ school to provide participants a relaxed environment. Additionally, in order to get accurate responses from participants, interview questions will be presented in Turkish, native language of all participants. The current study includes four different research tools for understanding systems thinking level of students. The tools are adapted from Assaraf and Orion which are word association, drawings, concept map and interview (2005). Data obtained from the interviews were analyzed in consideration of systems thinking skills defined in the research with using the rubric developed for this study. For analysis, each interview was transcribed, and transcriptions were reviewed number of times. Interview transcripts were used for coding, regarding the systems thinking skills. Coding was performed with the using the categories and definitions for each STS. For each systems thinking skill, three levels were determined; Level 1, Level 2 and Level 3. Necessary requirements to achieve each level were explained explicitly in the rubric. Before conducting the study, first, Human Subject Ethics Committee’s and Turkish Education Ministry’s permission were taken.

The arguments provided by Assaraf and Orion (2005) reveal that systems thinking seems to be an indispensable part of understanding complex systems. Therefore, inquiries about students’ system thing level provide information about how they can deal with complex systems. Many previous researchers concluded that students start maintaining systems thinking level at a lower grade and once they master them, they become successful at higher levels since each level serves as a basis for the next. However, most of the studies stated that students have difficulty to achieve higher level systems thinking level since they have trouble with complex systems. Studies showed that students have difficulties make a holistic understanding of the human body and the dynamic nature of the human body. Students’ sentences and drawings in this current study support previous studies. Participants accept human body as a system, but they have difficulty to identify hidden dimensions, and dynamic nature of the system. One of the students said that “Human body is a system because all parts have different function but same aim…”. The other student said that “We need to think human body as a whole system but it has many small systems. All systems interact with each other, and this interaction makes human a system.” This study which is conducted to 7th grade students who exposed the new curriculum will contribute literature about their systems thinking ability in this context. They are expected to show their understanding of complex system with its interaction. In the light of previous studies, it can be said that students have difficulty to achieve the third level of systems thinking ability which is implementation specifically, students’ understanding of homeostasis.

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