This article reports research concerning the definition of variables by pupils aged 12 to 14 during investigative work. The findings suggest that an increase in the complexity of an investigation lowers the ability to identify relevant variables as pupils start to lose track of the whole task. Substantive concepts intrude most obviously on the ability to dejine the appropriate dependent variable and to control variables. Pupils appear to have difticulty in realizing the advantages of defining the independent variable as continuous for the investigation as a whole. Possible explanations are considered and some practical ways of addressing this point of difficulty are put forward.
Investigative work is a distinct type of practical work in school science, characterized by the fact that an answer to a question cannot be taken for granted, and although the pupils may speculate or hypothesize about what the solution might be, they will need to carry out a practically based investigation to prove or disprove their hypothesis. 'The question which the pupil is invited to answer is either given to the pupils-for example, "Find out how the temperature of the water affects how quickly the sugar dissolves"-or developed by the pupils from a more open question-for example, "Which type of fuel is best?" or generated by the pupils themselves.
In the second example, the pupils can interpret besr in a variety of ways, such as duration of burning, time taken to boil a fixed amount of water, the rise in temperature of a fixed amount of water in a given time, the degree of smokiness, etc. The subsequent investigation consists of some kind of plan (written, oral, or implicit), an implementing or doing phase, which includes recording of some kind, and a final stage in which the pupil examines the data and arrives at a conclusion. No further instructions are provided, however; the pupils are left to come up with their own method of solution to the task. Such work now forms a key element in the U.K. National Curriculum for Science which began in 1989. The National Curriculum is a government-led initiative aimed at improving national standards in education by prescribing the content and subsequent assessment of the curriculum for all pupils ages 5 to 16.
In a recent publication (Gott & Duggan, 1995), we considered some of the theoretical issues underlying investigative work in science. We introduced the idea of distinguishing skills and the notion of concepts of evidence within the synractical structure of science. This structure can be seen as encompassing procedural understanding in the same way that the substantive structure encompasses conceptual understanding (or subject knowledge) (Figure 1 ). Further-