Categorizing players into respective age groups according to a cut-off date is typical of team sports (Baker et al., 2009). This process arises from the necessity of organizing the games for the purposes of competition among young athletes. This grouping leads to a situation in which players with even 12-month chronological age differences are competing against each other (Cobley et al., 2009). Consequently, there are considerable differences in the physical body structure of these athletes, which may hinder their development and promote disturbances in the process of sport talent identification (Williams and Drust, 2012). This situation results in the phenomenon of overrepresentation of players born in the quarter of a calendar year that is the closest to the cut-off date, called the relative age effect (RAE) (Delorme et al., 2010) .
One of the main predictors of success in basketball is height (Silva et al., 2013; Torres-Unda et al., 2013). Additionally, anthropometric basketball players' body measurements are a key factor in selecting their position on the team (Ben Abdelkrim et al., 2010a; Drinkwater et al., 2008; Koklu et al., 2011). Therefore, the advantage in motor skills of relatively older players may disturb the assessment of a player's performance potential (Lockie et al., 2014; Sisic et al., 2016).
The existence of the RAE in basketball has been confirmed in youth and professional sports (Arrieta et al., 2016; Chittle et al., 2016; Delorme and Raspaud, 2009). However, this phenomenon has not been observed among the players of the national teams during the Olympics (Werneck et al., 2016). The only exception was the national team of France, where the occurrence of RAE among the entire population of basketball players was confirmed by another study (Delorme and Raspaud, 2009). Additionally, the RAE has not been reported in the National Collegiate Athletic Association (NCAA) in the United States (Chittle et al., 2016). Nevertheless, the RAE in European basketball is a phenomenon with a long-term influence, resulting in the overrepresentation of judges and coaches born in the first quarter of a calendar year (Schorer et al., 2011).
Skewed distribution of dates of birth was observed at any positon on the court regardless of sex (Arrieta et al., 2016). However, a basketball player's position in the field is related to body height and age at peak height velocity (APHV) (Ben Abdelkrim et al., 2010a). Additionally, boys enter their APHV much later than girls (Carvalho et al., 2011). Furthermore, a strong relation between the RAE and players' performance, called the PIR (performance index rating) index (Torres-Unda et al., 2016), has been observed. The statistical formula for calculating the PIR index generally classifies the player's performance and his/her impact on the team's performance. However, the PIR index may not have been sufficiently adjusted to the specific characteristics of youth basketball.
A player's age at puberty is a key distracting factor in the process of basketball selection (te Wierike et al., 2015). It has been observed that the weight of a basketball may considerably change the shooting strategy of young players (Arias et al., 2012). Therefore, players with a physical advantage over their peers may make more three-point attempts. Furthermore, there is a shooting technique in which the hands are held up high among young players, which may hinder effective defense for shorter players (Arias, 2012). Consequently, shorter players may choose to shoot from distances closer to the basket, often giving up long-distance shots completely. However, there has been no research identifying the RAE phenomenon considering effectiveness with regard to a shot zone in relation to a player's position during a game.
To address this research gap, the objective of the article is to identify the RAE in youth basketball games in Poland while taking into consideration the age, sex and the players' match statistics. Additionally, the aim of this study is to determine whether differences in the body height of players are associated with the success of the team. We hypothesized that the phenomenon of RAE occurs in the Polish Championships, regardless of sex and age of the basketball players.
This study included 7268 young male and female basketball players from 591 teams registered in the official database of the Polish Basketball Association between 2013 and 2016. The analyzed players were from 14 to 22 years old and participated in the finals of the Polish Championships. The obtained data concerned the date of birth, body height and players' match statistics. The Central Statistical Office provided distributions of births in the Polish population (PP) between 1994 and 2003, which corresponded to the dates of birth in the analyzed groups. The data were obtained according to the Data Protection Act in Poland. All the research procedures were approved by the Research Ethics Committee of the University School of Physical Education in Wroclaw.
To identify the date of birth distribution, the players were assigned to the subsequent quarters of a calendar year: Q1 (January-March), Q2 (April-June), Q3 (July-September), and Q4 (October-December). Furthermore, the players were divided into two halves of a year: H1 (January-June) and H2 (July-December). The dates of birth of female and male populations in Poland between 1994 and 2003, which correspond with the dates of birth of the players participating in the finals of the Polish Youth Basketball Championships, were arranged in a similar way. Additionally, the players were assigned to three...