The field-weighted citation impact metric does not adequately account for publication age

Abstract To investigate the association between publication month and field-weighted citation impact (FWCI), a cross-sectional study was conducted using 2.1 million medical publications from SciVal, from the 10 countries with the most publications, during 2019–2023. Publications from January and December were excluded due to inaccurate recording of publication dates in SciVal. Quantile regression was used to estimate the association between publication month and FWCI quantiles (50th, 75th, 95th, and 99th percentiles) for 2019–2023 and each separate year. Median FWCI decreased, on average, by 0.030 per month (95% CI: 0.029, 0.031). The equivalent FWCI decreases for the 75th, 95th, and 99th percentiles were 0.067 (95% CI: 0.066, 0.068), 0.232 (95% CI: 0.227, 0.237), and 0.580 (95% CI: 0.553, 0.607), respectively. The median FWCI in November (0.46) was 39% less than the median FWCI in February (0.75). The strength of association between publication month and FWCI percentile was stronger for more recent publication years. When using data for 2023, the median FWCI decreased, on average, by 0.065 per month (95% CI: 0.064, 0.066), compared to 0.017 per month (95% CI: 0.016, 0.018) when using data from 2019. The median FWCI in November 2023 was 64% less than the median FWCI in February 2023, compared to 28% for the equivalent difference in 2019. These findings demonstrate a strong association between FWCI and the month of publication. FWCI is a biased indicator of research impact and needs to account for publication-age dependence by comparing articles from the same month, not only the same year.