Time-lag reporting bias in interrupted time series studies: a methodological study

Time-lag bias occurs when the speed of publication depends on the nature of the results (e.g., whether they support the authors' hypothesis). Time-lag bias has not been examined among interrupted time series (ITS) studies.

To investigate whether the time-to-publication of ITS studies with a protocol is influenced by their results METHODS: We searched eight bibliographic databases and 14 other sources for published protocols of ITS studies, up to 31 December 2022. We then searched for reports of results that matched these protocols by their primary ITS research question. The time-to-publication for each ITS study was calculated as the time between publication date of the report of results and that of the protocol or the end date of data collection period, whichever was later. In each report, all eligible results pertaining to the primary research question were classified as favourable or not favourable to the interruption, based on statistical significance and direction of effect. We estimated the median time-to-publication from a Kaplan-Meier curve of the rate of publication of results over time. To investigate time-lag reporting bias, we conducted multivariable linear regression to examine whether the percentage of favourable results per study was associated with time-to-publication.

We identified 120 ITS study protocols published between 2009-2022, of which 44/120 (37%) had at least one matched report of results, with a corresponding 860 results. Among these 44 protocols, the median time-to-publication was 2.0 years (95% CI 1.6 to 2.6) from the protocol's submission. The median percentage of results favourable to the interruption per study was 20% (interquartile range [IQR] 5% to 50%). For each percentage point increase in the percentage of results favourable to the interruption per study, the time-to-publication increased by 3 days (95% CI 5 days earlier to 10 days later; p=0.505).

We identified that very few protocols for ITS are published. In those published, there was uncertainty about whether studies reporting results that are favourable to the interruption are published sooner. Publication of protocols for ITS studies needs to become a normative practice.