The procedural understanding of university students at the freshman level, prior to instruction, has been investigated in the context of experimental work in physics. A written instrument was used to explore students' ideas about data collection, data processing, and data comparison; in particular, the need to repeat measurements and the implications of the scatter associated with numerical and graphical data. Two types of reasoning used by students in the laboratory, viz. point and set reasoning, are proposed and used to classify students' responses. The findings show a high degree of consistency of these modes of reasoning across the experimental phases of data collection and data processing. A framework is suggested from which a laboratory teaching program may be developed.