Abstract
Comparative judgement (CJ) provides methods for constructing measurement scales, by asking assessors to make a series of pairwise comparisons of the artefacts or representations to be scored. Researchers using CJ need to decide how many assessors to recruit and how many comparisons to collect. They also need to gauge the reliability of the resulting measurement scale, with two different estimates in widespread use: scale separation reliability (SSR) and split-halves reliability (SHR). Previous research has offered guidance on these issues, but with either limited empirical support or focused only on education research. In this paper, we offer guidance based on our analysis of 101 CJ datasets that we collated from previous research across a range of disciplines. We present two novel findings, with substantive implications for future CJ research. First, we find that collecting ten comparisons for every representation is generally sufficient; a more lenient guideline than previously published. Second, we conclude that SSR can serve as a reliable proxy for inter-rater reliability, but recommend that researchers use a higher threshold of .8, rather than the current standard of .7.