1. Two-tier Diagnostic Test

The other approach can be used to investigate students’ understanding is two-tier diagnostic test which was developed by David F. Treagust. This approach is powerful to encourage students’ analytical thinking on understanding the concepts. In addition, there are varied format of this instrument (Mann & Treagust, 1998), but basically the first tier of each item in the test is a multiple choice of possible answers which relates to the problem statements. The second tier of each item is composed of a multiple choice set of reasons for the answer related to the first tier which consist of one scientific concept and others alternative conceptions.

Furthermore, this instrument can be used for formative assessments, assessing students’ understanding and encouraging them to think about the concepts rather than memorize the facts (Treagust & Chandrasegaran, 2007) which could lead to interesting discussion and meaningful information of students’ understanding. Moreover, since current assessment not provide valid measures of students’ knowledge and encourages discussion, this instrument could be a solution. It is because this instrument requires the explanation and answers which is given to assess students’ knowledge. According to Taber (2000) as cited in Levy Nahum, et.al. (2004), students’ explanations of scientific concepts is evidence of conceptualise process. Two-tier diagnostic instrument could be used as effective way of assessing meaningful learning among students. As a result the information could be used to improve teaching and address students’ alternative conceptions.

In addition, according to Treagust (1988), there are three main procedures to develop this instrument:

1. The content is defined by the identification of propositional content knowledge statements of the topic to be taught (using the concept map)

2. Information about students’ conceptions which could be found through literature review, students’ free explanations, and unstructured interview)

3. Development the two-tier multiple choice diagnostic items. First tier consist of content questions which could be 2-4 choices. The second tier consists of four possible answers given to the first part. Then, correct answers for both of tiers are correct

Moreover, there are several studies which have been investigated in chemistry: (1) chemical bonding (14-16 years students, first tier True or false, second tier explanation, (2) chemical reactions using multiple representations (grade 9) (enhance students’ ability to describe and explain some chemical reactions), (3) qualitative analysis, (4) ionization energies of elements, (5) acids and bases, and (6) states of matters (Treagust & Chandrasegaran (2007). However, according to Treagust & Chandrasegaran (2007), raw score on the test could underestimate students’ knowledge, for example students who looked on deeper meaning could give the answers which are categorized as wrong answer. Moreover, sometimes it becomes more tests taking skills rather than the extant knowledge. This is an example of process of development two-tier diagnostic instrument on chemical reactions:

Stage 1. Defining the content

Chemical reactions topic is integrated in secondary school in Indonesia consist of chemical change (law of conversion of mass), equation (products and reactants), energy (exothermic and endothermic), balance the equations, and type of reactions.

Stage 2. Investigate information about students’ conceptions

There are several studies on students’ alternative conceptions of chemical reactions:

1. Students find difficulties on balancing the chemical reaction equations on the concepts of subscripts and coefficients, (Sanger, 2005)

2. Students’ alternate conceptions in chemistry on the conservation of mass, molecules, and atoms during a chemical reaction is “the total number of molecules is also conserved in a chemical reaction”, Mulford and Robinson (2002)

3. Students think that boiling is part of chemical reactions because of the bubble formations (gas) and students also find difficulties to understand different types of chemical reactions and how it’s happened which is related to chemical bonding (my experiences on teaching chemical reactions)

Step 3. Develop the two-tier multiple choice diagnostic items

Example Items 1 are modified version from existing instruments. Item No. 2 is developed by Chandrasegaran, Treagust, & Mocerino (2005) :

1. Salt (NaCl) is added into the water, and then it is heated. After couple minutes the salt can longer be seen, bubbles come out and the water will taste salty because the chemical reaction is happened.

a. True b. False

Reason:

1. The heat water break up the chemical bonding in the salt compound into Na and Cl

2. It is only solution process, because salt is soluble in the water

3. The chemical reactions is happened because the bubbles is indicator of chemical reactions

4. Water molecules surround the salt molecules and break up the salt into smaller particles

5. __________________________________________

2. The symbol for the magnesium present in magnesium ribbon is

A. Mg b. Mg²⁺

Reason:

1. Magnesium has a charge of +2

2. The Magnesium atom is highly reactive

3. The Magnesium atom has positive nucleus

4. The particles in magnesium are neutral atoms

Furthermore, this instrument is important for my pre-service teachers as well as the teachers not only to assess their alternative conceptions but also to investigate their students’ understanding. As a result, the information from this instrument could be used to choose the appropriate teaching approaches to deal with students’ alternative conceptions.

Conclusion

Firstly, probing students’ understanding in chemistry need to be concerned by teachers, not only to obtain information about students’ conceptions, but also as starting point to choose appropriate teaching strategies. Information of students’ understanding could help teachers to understand the problems which are held by the students to understand the concepts.

Secondly, questioning and interview approach are powerful to probe students’ understanding through depth probing. However, these approaches require much time and substantial skills. In addition, concept map and concept cartoon are useful to stimulate students’ attention and motivation. These approaches could be applied within different procedures and purposes. The two-tier diagnostic test is not only to probe students’ conceptions and understanding, but also could be used as formative assessment and discussion topic.

Thirdly, all approaches could be used to probe students’ understanding, stimulate the discussions, motivate students’ learning, and create meaningful learning experiences. However, each approach has own characteristic, strength and limitations, therefore, teacher may choose the appropriate approach to be used in the classroom.