This paper reports some of the findings of the Procedural and Conceptual Knowledge in Science (PACKS) project. It documents children's ides about the reliability of experimental data, an important element of their understanding of science procedures. These ideas were elicited using a written survey instrument completed by over 1000 students aged 11, 14 and 16, chosen to be representative of the full ability range. The results show progression with age in understanding of empirical data. A range of ideas about the function of repeat measurements, how to handle repeat measurements and anomalous readings, and the significance of the spread of a set of repeated measurements emerge and are discussed.