Content analysis of undefined arbitrary unit labels in scientific figures

Purpose: In scientific notation, “a.u.” denotes atomic units, but in figure labels it is also widely used to mean arbitrary units. This technically incompatible dual usage motivated this study, which examined the prevalence, disciplinary distribution, and editorial handling of the use of “a.u.” to denote arbitrary units of intensity in peer-reviewed publications.Methods: A cross-sectional bibliometric content analysis was conducted using 1,915 research articles published between 2020 and 2024 in six representative high-impact journals. Articles with intensity axes labeled “a.u.” were identified and coded by field, context, and whether the intended meaning was explicitly defined.Results: Of the 1,915 articles sampled, 1,140 (59.5%) contained at least one figure with “a.u.” on an intensity axis. Among these articles, only 14.3% explicitly clarified that the label denoted arbitrary rather than atomic units. Significant inter-journal variation was observed: <i>Optics Express</i> had the highest prevalence of “a.u.” intensity axis labeling (71.1%) and the lowest clarification rate (8.1%), whereas <i>PLOS One</i> had the lowest prevalence (41.9%) and the highest clarification rate (25.7%).Conclusion: The conflation of “a.u.” as atomic units with “a.u.” as arbitrary units is a pervasive, editorially tolerated notational ambiguity that can undermine scientific precision and reproducibility. The unambiguous alternative “arb. units” should be adopted, and explicit editorial guidelines should be implemented.