42 Chapter 3 Materials and methods Ethical approval All procedures used in the study were approved by the Local Ethical Committee of the University Medical Center Groningen, University of Groningen, The Netherlands (201900334) in the spirit of the Helsinki Declaration with a waiver of the requirement for informed consent of the participants given the fact that the study involved the analysis of publicly available data. Data collection The swimmers we selected for this study were international male and female swimmers with performance data on the 100m freestyle long course event. We chose this event because it is considered as the key distance in competitive swimming. It has been on the Olympic program since 1904 (men) and 1912 (women) and is characterized with the largest number of world-wide participants. Moreover competition starts from an early age on and the word-wide competition level is high for both male and female swimmers (FINA, 2019; Swimrankings, 2019). Performance data (in terms of swim times) was obtained from Swimrankings (2019), a recognized public data source which records international swimming race results. Performance data were collected from 88 countries across different parts of the world including Africa, America, Asia, Australia and Europe. We collected all available 100m freestyle long course results from Swimrankings’ database, which initially resulted in 2,864,4481 observations between 1993 and 2019. Data processing For the purpose of the present study, we transformed the structure of the dataset. Starting with individual competition observations (each observation e.g. swim performance stored into a unique row), we restructured the dataset in individual season observations (two observations e.g. swim performances stored in one row). The two observations we stored in one row were the first swim performance of the swimming season and the best swim performance of the swimming season. All other performance data within the season were discarded from further analysis. Performance data from the 1st of January 2008 till the 1st of January 2010 were excluded from analysis (we exclude full-body polyurethane swimsuits (Toussaint et al., 2002; Tiozzo et al., 2009; Tomikawa and Nomura, 2009). Swim performances over 180 seconds were excluded from analysis to ensure a representative dataset. Based on swim dates, performance data were classified in swimming seasons. Each swimming season officially starts on the 1st of September of a calendar year and ends on the 31st of August of the next calendar year (FINA, 2019). Swimmers were classified in age categories based on their
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