Readers' attention to shorter versus longer abstracts of systematic reviews: a randomised controlled trial.

Objectives: First, investigate whether a long compared with a short abstract decreases readers' attention. Second, investigate differences regarding perceptions of informativeness, accuracy, attractiveness and conciseness.

Design: Two-arm, single-blinded, parallel-group, superiority randomised controlled trial with 1:1 allocation.

Setting/participants: Researchers worldwide who indexed any type of systematic review in PubMed with an English abstract between 1 January 2024 and 26 March 2024.

Interventions: Researchers were randomly assigned to two groups. Both groups received the same cover letter by email with a link to our survey, which was assigned to either the short (277 words) or long abstract (771 words) of the same systematic review published in two different journals.

Main outcome measures: Primary outcome was the proportion of trial participation after reading the abstract, indicating readers' attention. Secondary outcomes were researchers' perceptions of four indicators of a well-written abstract (informativeness, accuracy, attractiveness, conciseness), and general abstract characteristics.

Results: A total of 5397 authors were randomly assigned to the short (n=2691) or long abstract (n=2706). Trial participation did not differ between groups (37.8% vs 35.0%; p=0.1935). While the short abstract was considered more attractive (60.5% vs 46.6%; p=0.0034) and concise (82.3% vs 37.9%; p<0.0001), the length had no impact on its informativeness (85.5% vs 91.2%; p=0.0594) and accuracy (80.2% vs 86.3%; p=0.0868). Regarding general abstract characteristics, 76.0% preferred a maximum length of 250-300 words, nearly all a structured format and about half supported reporting funding and registration information.

Conclusions: Abstract length had no impact on readers' attention, but short abstracts were considered more attractive and concise. Guidelines like PRISMA-A should recommend a range of 250-300 words for abstracts, allowing authors to include key information while prioritising clarity and precision. With authors considering information on funding and registration as important, journals should update their author guidelines to include these by default.

Trial registration number: NCT06525805.FundingNone.