Performance assessment in the UK science General Certificate of Secondary Education (GCSE) currently relies on pupil reports of their investigations. These are widely criticized. Written tests of procedural understanding could be used as an alternative, but what exactly do they measure? This paper describes small-scale research in which there was an analysis of assessments of pupils' GCSE scores of substantive ideas, their coursework performance assessment and a novel written evidence test. Results from these different assessments were compared with each other and with baseline data on CAT scores and pupils' attributes. Significant predictors of performance on each of these assessments were determined. The data reported shows that a choice could be made between practical coursework that links to "behaviour" and written evidence tests which link, albeit less strongly, with "quickness". There would be differential effects on pupils. (Contains 1 note, 10 tables and 6 figures.)