This article reports research concerning the definition of variables b
y pupils aged 12 to 14 during investigative work. The findings suggest
that an increase in the complexity of an investigation lowers the abi
lity to identify relevant variables as pupils start to lose track of t
he whole task-Substantive concepts intrude most obviously on the abili
ty to define the appropriate dependent variable and to control variabl
es. Pupils appear to have difficulty in realizing the advantages of de
fining 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.