Dear Editor,
We read with great interest the article published by Kohli et al.[1] titled “Incidence of ocular and systemic diseases affecting visual function among state bus drivers” published in Indian Journal of Ophthalmology. The authors have done a commendable job in highlighting the visual acuity of bus drivers, which is an important public health issue regarding road traffic accidents. However, there are a few concerns regarding this study.
It has been mentioned by the authors that this was a retrospective observational study of the year 2019 where bus drivers’ annual ocular examination records were reviewed and the data retrieved was analyzed. However, a review of hospital-based records (which is a one-time annual activity) cannot provide incidence. Incidence, a rate, is defined as the occurrence of new cases in a population for a specified period of time. Without following the bus drivers for a specific time period for occurrence of event (ocular diseases), the incidence rate cannot be provided.
In a stricter sense, incidence is a measure of the probability of developing a particular condition during a specified period for the individuals in the population under observation.[2] Also, in Table 1, the authors presented visual function–threatening systemic diseases’ data as incidence, which should have been frequency and percentage. In the Results section, the authors have mentioned “The incidence of DR among drivers with DM was 21.3%,” that is, as a percentage. Incidence is a rate and it is expressed as per thousand, per lac, or per million per year.[3] As this was cross-sectional data with no follow-up period, the term “prevalence” should have been used ideally. As already stated above, incidence is commonly presented as person-years or event-years but not as a percentage.[3] The same analogy can be applied to the title of the study too.
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References
- 1.Kohli P, Babu N, Mishra C, Damodaran S, Bhavani S, Kumar M, et al. Incidence of ocular and systemic diseases affecting visual function among state bus drivers. Indian J Ophthalmol. 2021;69:2625–8. doi: 10.4103/ijo.IJO_76_21. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Wallace RB. Epidemiology and Public Health in Maxcy, Rosenau and Last:Public Health and Preventive Medicine. 15th ed. Mc Graw Hill Publishers; 2008. p. 19. [Google Scholar]
- 3.Park K. Incidence in Park's Textbook of Preventive and Social Medicine. 24th ed. Banarsidas Bhanot Publishers; 2017. p. 65. [Google Scholar]