 Realist accounts of measurement are largely formulated in opposition to strong versions of operationalism and conventionalism, which dominated philosophical discussions of measurement from the 1930s until the 1960s. In addition to the drawbacks of operationalism already discussed in the previous section, realists point out that anti-realism about measurable quantities fails to make sense of scientific practice. If quantities had no real values independently of one’s choice of measurement procedure, it would be difficult to explain what scientists mean by “measurement accuracy” and “measurement error”, and why they try to increase accuracy and diminish error. By contrast, realists can easily make sense of the notions of accuracy and error in terms of the distance between real and measured values (Byerly and Lazara 1973: 17–8; Swoyer 1987: 239; Trout 1998: 57). A closely related point is the fact that newer measurement procedures tend to improve on the accuracy of older ones. If choices of measurement procedure were merely conventional it would be difficult to make sense of such progress. In addition, realism provides an intuitive explanation for why different measurement procedures often yield similar results, namely, because they are sensitive to the same facts (Swoyer 1987: 239; Trout 1998: 56). Finally, realists note that the construction of measurement apparatus and the analysis of measurement results are guided by theoretical assumptions concerning causal relationships among quantities. The ability of such causal assumptions to guide measurement suggests that quantities are ontologically prior to the procedures that measure  them.[14]