Discussion

This study established the pattern of researcher performance over an entire career in a medical context. There was a pronounced difference in scientific productivity between two distinct populations of researchers. Compared to full professors, non-academic physicians had a poor capacity to publish, indicating a low performance when medical doctors have limited time, poor incentives or no interest in research. The publication volume among full professors evolved in three successive phases: the initiation phase with a dramatic 100-fold increase in scientific production before 35 years of age, the maturation phase with a doubling in production between 35 and 50 years of age, and the stabilisation phase with constant production followed by a potential decline at career end. The performance curve for non-academic physicians showed the same change with a less pronounced dynamic and a gradual downturn in the slope of production improvement during their career. Furthermore, the scientific production of researchers was strongly influenced by their birth cohort, supporting the hypothesis of a secular trend. There was a significant increase in publication volume among the research community born more recently compared to older cohorts. This effect was observed among both full professors and non-academic physicians, suggesting an increasing production over time as the generations succeed one another.

The main strength of this work is its longitudinal design that provided a valid picture of performance curves for individual researchers by exploring the change of scientific production over a career according to their age, academic position and birth cohort. The chosen GEE model was appropriate to evaluate the mean performance trajectories according to various determinants, even though this approach did not allow comparison between models. Outcome measurement based on the SIGAPS bibliometric system was accurate because this required individual approbation by researchers with incentives to validate their publications in the system.12 Human resources data were also exhaustive and of high quality because this information was critical for payment of salaries.

The main study limitation is the local context that may affect the generalisation of results. In particular, the absolute number of publications by researchers may have been influenced by how the research teams and disciplines were locally organised. Although these findings would deserve to be replicated using a multinational community of researchers, we assume that the pattern of the performance curves highlighted and the relative differences between academic positions and birth cohorts would be identical in a more general context. Furthermore, the gender of the researchers was not considered in determining performance curves for robustness considerations, due to the small number of women in several stratums related to academic position, birth cohort and discipline. Resolving this issue would require a larger cohort of researchers to be investigated. Another limitation relates to the absence of consideration of the total number of co-authors in analyses to control for opportunistic authorship strategies.14 Collaborations within and across research teams or in some disciplines that systematically include an important number of authors with limited contributions can trigger a spurious inflation in the volume of publications and an overestimation of scientific production at the individual researcher level. This aspect could not be evaluated in the present study because the number of co-authors in each paper was not available, which limited the analysis to full counts of publications instead of fractional counts. However, our sensitivity analysis based on the number of publications as first/last author revealed unchanging results for most findings. Finally, volumetric analysis based on referenced papers does not necessarily reflect when a researcher has full capacity to make a scientific breakthrough during his/her career. Identifying qualitatively the ground-breaking nature and potential impact of research findings beyond the state of the art (ie, novel concepts across disciplines with high gain for scientific community and public health) requires another approach. This may reveal a different pattern of individual performance curves with an innovation peak occurring earlier during the scientific careers of young researchers. Jointly with bibliometric evaluation, researcher performance could also be assessed using other aspects of scientific production. Active collaboration with international research networks or the mentoring of future researchers would make sense for the most experienced researchers in the last part of their career.15

The definition of ‘scientific productivity’ in terms of volume is subject to much debate in the research community because this is a complex notion, the measurement of which can include a wide range of documents including not only publications in peer-reviewed journals but also books, reports, conference abstracts, oral communications or filed patents.16 To date, there is no consensus for a gold standard in measuring scientific production and a wide range of criteria exists in the literature.4–9 In this study, the basic criterion of publication volume was refined to reflect the substantial contribution of the researcher in scientific projects as first/last author and the visibility of his/her own works in high impact journals. We identified the same determinants of scientific productivity that have been reported in other investigations conducted worldwide, including age, discipline and academic position.5 ,8 ,10 While a similar pattern of performance curve by age was found in the literature and corroborates our findings, this result should be cautiously interpreted because it could reflect exogenous factors occurring at career milestones, such as shifts in job security or responsibilities. Previous work found a more important decline of scientific production at the end of career because publication volume was not adjusted for birth cohort and older researchers belonged to the oldest generation.7 One reason for this age-related decline in scientific achievement might be that a full professor cannot maintain a high level of scientific production passively by accumulating experience, which raises concerns about motivation throughout a career that extends over several decades. In many European countries, academics need publications under their names if they want to reach the rank of full professor, but this pressure to publish disappears once they have reached this goal. The average age for achieving the position of professor at our institution was closer to 45 than 55 years. A potential explanation for the absence of a decline in performance immediately after appointment is that personal status within the academic system also relies on the research funds one acquires, and because even senior professors have to continue publishing if they want to be respected by their colleagues and to continue mentoring top graduate students.17 This late peak and minimal decline in performance at the end of a researchers career is consistent with previous works.18

The shape of the observed performance curve for medical researchers seemed close to the ‘conceptual’ curve proposed by Ericsson in other fields such as chess or music.11 However, this ‘conceptual curve’ had no strong empirical basis and was directed at performance in well-defined task domains, as evidenced previously for systematic care delivery.19 ,20 Considerable evidence shows that creative productivity does not necessarily work the same way for researchers, yielding different expected longitudinal functions.21 There are fundamental shifts in the lifecycle of research productivity, and the frequency of great achievement at a young age would be more a function of time than field. Indeed, independent associations have been found between age dynamics within fields and both the prevalence of theoretical work and the measures of the stock of foundational knowledge.22 In the same way, the generational increase in productivity has been well established in various industrial sectors.1 ,2 ,23 Beyond the broadening in available space for publishing in biomedical journals, the effect of the birth cohort may reveal a growing productivity of researchers whose practices are impacted by more incentives to publish. Indeed, public institutions and research funding tend to prioritise career advancement based on metrics reflecting their publications in peer-reviewed journals.24 It is of note that this secular trend was found for overall publication volume and not the number of publications as first/last authors, which may indicate changing practices across generations towards more collaborations.25 Additionally, assuming that young researchers are most likely not independent and are collaborating with senior researchers already in their maturation or stabilisation phases, this may also represent a virtuous circle with increasing publication capacity over generations. Variations also exist within each generation and researchers who are highly productive in their 30s are also likely to be much more productive in their 60s than researchers who are not very productive at a young age.

Based on routinely collected data from a hospital information system over 20 years, this study established an accurate curve of individual performance among medical researchers during their career. Using this curve to evaluate researchers integrates the need to consider their personal characteristics for a fair interpretation of their scientific production. Indeed, it would be inappropriate to expect from a physician who has just started his/her training to perform similarly as a professor at the peak of his/her career. Each researcher can now follow his/her publication volume over time depending on what is expected in view of his/her experience, academic position and year of birth. Such an approach, both dynamic and researcher-centred, should enable realistic goals to be set by researchers to improve or maintain their performance throughout their career. A further implication regards the organisation of research at the macro level of university hospitals. To date, most of publications are produced by a limited number of professors, while there is a modest contribution of non-academic physicians to research effort in spite of representing most of medical workforce in university hospitals. Rethinking the missions of all medical doctors towards an enlargement of scientific prerogatives would represent a substantial investment at the level of each institution in favour of progress in global knowledge.

To this end, we need tangible elements about the optimal balance between research, teaching and care activities that can be performed by the same person. Although clinical activities may catalyse the emergence of original research ideas, overwhelming investment of medical doctors in patient care reduces even more their time dedicated to science. Spending adequate time in research activities is essential to allow principal investigators to lead creative and well-designed research projects. Better understanding of the effect on scientific production of time spent exclusively for research purposes compared to time spent in administrative tasks or patient care would be of interest to medical researchers and their host institutions. This poses the question of how to prioritise the time of medical researchers to increase their scientific production and the chance of major discoveries without compromising patient care.