Abstract
The ability to select the correct dose when using pen-injectors is both a prerequisite for accurate delivery of medicinal products and a high priority for many users when choosing their treatment. A large body of published work has explored how different pen-injector designs affect safety and effectiveness. However, despite the importance of dose selection, publications on the design of dose dials remain scarce. Many pen-injectors enable users to vary the dose by twisting a dial and the selected dose is shown on an analogue scale drum. Thus, visual inspection is necessary to confirm that the correct dose has been selected, and it is crucial that the visual design of the scale drum accommodates selection of all possible doses. However, it is not always possible to represent all doses numerically on the scale, and users might need to rely on markings between numbers. This paper details an online user study (n=200) exploring how the readability of doses displayed on scale drums is affected by different scale drum designs. Specifically, we explored designs where (1) the scale is based on even or odd tens in increments of 20 (i.e., either 0, 20, 40, 60, etc. or 10, 30, 50, 70, etc.), and (2) the dose is represented by a numerical value or a line marking between two values. The study did not reveal differences between scale drums displaying odd and even tens, which might indicate that the two number representations are similar in terms of readability. However, the results suggest that it might be harder to read the current dose from a line marking than a number. Taken together, these results indicate that it is relevant to consider whether common doses are administered in an even or odd number of units when designing scale drums. Moreover, the results indicate that even-numbered endpoints on odd scales might be harder to read, and none of the compared designs were able to alleviate this problem completely. Finally, even though the online format limits ecological validity, the study yielded valuable insights about the relative readability of the compared design. Thus, the study indicates that surveys may be used as an inexpensive and effective supplement to physical studies.