The relative age effect in Dutch youth Judo

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6/28/2016

The Relative Age

Effect In Dutch

Youth Judo

EFLP 1

Karen Stevenson

UNIVERSITY OF AMSTERDAM STUDENT NUMBER 10194525

SPECIALIZATION ECONOMICS & FINANCE

FIELD LABOR ECONOMICS

NUMBER OF CREDITS 12

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Statement of Originality

This document is written by Student Karen Stevenson who declares to take full responsibility for the contents of this document.

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Abstract

This thesis analyzed the impact of month of birth on being successful at the highest level of Dutch youth judo, known as the relative age effect (RAE). Successful was measured by winning a medal at Dutch national championships between the years 2000-2012 in the age categories under fifteen years (N = 886) and under seventeen years (N = 829). Data was obtained from judoinside.com. Analyzes were made on age category, gender and age category and gender combined. The months of birth were classified into four quarters and χ2 goodness of fit tests was performed to support the results. RAE was found in both age categories. RAE was found for male athletes (N = 883) but was not found for female athletes (N =832).

Literature Review

Earlier studies revealed that there is a strong correlation between academic success and the age of entry of a person (Davis, Trimble & Vincent, 1980). Later studies focused on children born in the same year but born in different months (Diamond, 1983). Both studies reveal that the older children performed better than the younger ones. The concept is called ‘relative age’. This means the

difference in age between two children in the same academic year due to the cutoff date. Although the children are in the same class they could have an eleven month difference in age. This difference in age has consequences because the older child is often more developed what is called ‘relative age effect’ or RAE.

Also in sports age groups and cutoff dates are used. The goals of age categories are to provide training that is in line with a child its mental and physical development, making competition fair and prevent big differences so chances on success are as equal as possible (Helsen, Starkes & Van Winckel, 1998). But there still remains an age difference within these groups, which suggest there would be a relative age effect. The most used cutoff date in sport is 1st of January. The children born early in the year will have an advantage compared to a late born child. The development of children is an ongoing process and in a few months some relative important and big changes can happen to an individual (Grondin, Deschaies & Nault, 1984). Therefor the older child has already more

experience and is further developed physically and psychologically (Barnsley , Thompson & Legault, 1992). Because of this developmental advantage they are more likely to get selected for the higher teams or win more prices in individual sports. Getting selected and winning of prices likely stimulates them to continue and to get more attention from the trainers for the same reason (Helsen et al., 1998). More attention and specialized training gives them more opportunity to develop their skills, this will make the difference between otherwise similar persons even bigger. Previous studies show that there is a greater chance of becoming an elite athlete if an individual is born early after the cutoff date (Baker, Schorer & Cobley, 2010; Musch & Grondin, 2001).

RAE in sports

The first link between unequal distribution of month of birth and participating in sport and/or in getting results was found in amateur youth hockey (Grondin et al., 1984). Numerous studies have looked into the existance of RAE at professional level in different types of sport and age categories after Grondin. Barnsley, Thompson & Barnsley(1985) proved RAE is significant in ice hockey. A few years later Barnsley et al. (1992) also proved RAE in football. Other studies reveal the same for more team sports for example soccer (Helsen et al., 1998; Delorme, Boiché & Raspaud, 2010; Vincent & Glamser, 2006) and basketball (Delorme & Raspaud, 2009). But it is also proven in individual sports,

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4 like youth winter sports (Raschner, Müller & Hildebrandt, 2012; Müller, Hildebrandt, Schnitzer & Raschner, 2016), tennis (Edgar & O’Donoghue, 2005; Baxter-Jones, 2005) and swimming (Baxter-Jones, 1995). But not in all types of sport RAE is found. Van Rossum (2006) states that RAE is often found in sports where physical aspects, such as strength, weight and height, play an important role. However in sports that depend mostly on technical ability, RAE will not be proven. The same Van Rossum (2006) did not find RAE in Dutch pre professional dancing and also for gymnastics (Baxter-Jones, 1995) and golf (Nakata & Sakamoto, 2011) it is not proven. The study from Müller et al. (2016) also supports this. At the youth European winter Olympics of 2015 RAE was found in both the competing athletes as well as in the medal winners for all strength and endurance sports, but not in the technical sports.

Extra factors that contribute to the existing of RAE

According to Musch & Grondin (2001) RAE is a combination of factors. If RAE was only based on physical development, RAE should decrease with maturation of children and not even be there at senior level. There are studies on RAE that already show this is not always the case. In the study of Barnsley & Thompson (1988), RAE did not exist at the youngest level tested and increased at older age categories. The level of competition and popularity of a sport are factors that influence RAE (Delorme et al., 2011). ‘‘The larger the pool of potential players, for a given sport in a given category, the strongest the resulting RAE should be’’ (Musch & Grondin, 2001, p. 154). This is also shown in earlier studies. Delorme (2011) looks at Canada to make a comparison between the country its most popular sport ice hockey and the far less practiced volleyball. This is underpinned by the researches of Barnsley et al. (2005) that shows high RAE for ice hockey and Grondin et al. (1984) that finds only weak RAE for volleyball.

Another important factor that causes RAE is selection of talent by physical development at young age (Delorme, 2011). Talent is equally distributed over the year and often the most developed ones at young age are not the most talented. Both Andersen, Houlihan & Ronglan (2015) and Baker, Cobley & Schorer (2013) state that many talented children are ‘wasted’ by selecting on physical performance at young age. This influences the motivation to continue for the ones that perform well and frustration from failure and unequal training opportunities for the ones that do not make it to the higher teams. Several studies show that the dropout rate is higher on late born children (Helsen et al., 1998; Delorme et al., 2011; Delorme et al., 2010).

Solutions to prevent RAE

There also have been some studies that look into solutions to prevent RAE or at least decreasing it. The main solution is raising the awareness of the existence off RAE (Pierson, Addona & Yates, 2014; Baker et al., 2013). By rising the awareness of trainers there is a good chance that trainers will look different at young talent. Both studies suggest that trainers should be looking at the potential rather than just looking at physical development and who is the strongest and furthest now. Look at how children move, their natural skills and how quick they pick up new stuff. The research of Furley & Memmert (2016) looked into the theory ofgrounded cognition, preconceptions, and found that in soccer and baseball body size and talent are automatically associated with each other. Where taller players were associated with positive qualities and smaller players were perceived as less talented. This suggests that even if there is awareness from trainers this does not take away this subconscious association that talented players are the taller players. Andersen et al. (2015) looked into the development of training for young children in Sweden. Sweden changed the system of education in ice hockey youth. This is no more a physical selection at young age but now focuses on having fun and giving every child the same training to obtain certain skills. This system resulted into a significant raise in Swedish ice hockey players playing in the National Hockey League (NHL) in North-America.

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5 The percentage of Swedish players in this competition doubled since 2000. With 9,1 percent of the players Sweden was the third supplier of players after Canada and the United States in the season 2015-2016 (NHL, 2016). Pierson et al. (2014) suggests even that late born children should be provided with additional support at young age only. The study showed that the additional support for late born children, combined with switching the cutoff date each year, was proven to reduce RAE by 96%. The rotation of the cutoff date was between 1st of January and 1st of July. Musch & Grondin (2001) even suggest a rotation by quarter but did not test this. In the same research Musch & Grondin mention weight classes as a solution because children in the same weight should not differ too much in physical conditions. Further examination if weight classes can be a solution will be considered in the next paragraph.

RAE in combat sport

As mentioned above, Musch & Grondin (2001) suggest that an option to prevent RAE can be weight classes. So to see if it is a solution there should be analyzed if there is RAE in sports with weight categories. In combat sports weight categories are often used. There have been studies that look into combat sports that use weight categories. Delorme (2014) looked into French boxing at both

amateur and professional level. Both men and women were analyzed and the conclusion was that there was no RAE at any level of French boxing. Also for taekwondo at Olympic level RAE is not found, but it must be noticed that the sample size in this study was small (Albuquerque et al., 2012). But there are also combat sports with weight categories where RAE is proven albeit to a certain extent. For Olympic judo tests where done on different specifics: sexes, weight categories and medalists (Albuquerque, et al., 2015). The outcome was that there was only RAE in male

heavyweights and medalists. More RAE was found in Olympic wrestling (Albuquerque et al., 2014). It is proven in all styles and under medalists, but only for men and not for females. The possible

explanation for RAE within judo and wrestling is that these sports seem to require higher anaerobic energy but also body size and strength matter rather than the sports that focus on punching and kicking your opponent where no RAE was found (Albuquerque et al. 2015). So, weight classes may make the differences smaller but cannot prevent RAE in all cases.

Male vs female

Certain studies noticed a difference in the level and some even in the existence of RAE between males and females. For males RAE is found very often, especially in sports where physical aspects play an important role, as mentioned earlier. For females it is only found in some cases. Leaving sports that required high technical skills out, RAE for was found in basketball (Delorme et al., 2011) and tennis (Edgar & O'Donoghue, 2005). At first in soccer only a small RAE was found for women, where in the same study for male players a strong effect was found by Vincent & Glamser (2006). Later Delorme et al. (2010) found a strong effect for female as well. On the other hand it is not found in boxing (Delorme, 2014), taekwondo (Albuquerque et al., 2012), the youth winter sports (Müller et al., 2016), wrestling (Albuquerque et al., 2014) and judo (Albuquerque et al., 2015). In the last 3 cases RAE was found for male athletes in the same sport. Most studies contribute the difference in finding RAE in the males division but finding weaker or even no RAE in the female division to the fact that women mature in a different way (Baxter-Jones, 1995). For girls puberty happens on average earlier than for boys. Puberty of girls appears between the age of twelve and fourteen and for boys

between thirteen and fifteen years old (Musch & Grondin, 2001). As mentioned before the way the maturing takes place is different, for girls it develops more gradually than for boys, who often have a growth spurt. Children, both boys and girls, whose puberty starts relative early even have a bigger advantage (Baxter-Jones, 1995). Another explanation given is that men have different motivations for sports and exercising than women. Men enjoy the competitive part of sports more, where

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6 women biggest incentive is to keep their body in shape and prevent overweight (Kilpatrick, Hebert & Bartholomew, 2005). It is commonly known that in some cultures women are still suppressed and not allowed to participate in any sport and as mentioned above higher popularity leads to stronger competition and contributes to the existence of RAE.

Dutch youth judo

This thesis will look into the possible existence of RAE in youth judo in the Netherlands. It will look into different age categories because as mentioned above in most sports the effect is decreasing at older age. It will also make a comparison between boys and girls, since RAE is proven for males most of the time and in fewer cases for females. The Dutch youth judo system uses 1st January as cutoff date. For young judokas there are many open tournaments where everybody can compete. In the youngest age categories, under eight, under ten and under twelve years old, sometimes the color of the belt is used to exclude the more developed judokas, but most competitions are based on open registration. The U15 category is the youngest category where Judo Bond Nederland, the Dutch judo federation, organizes national championships. In both U15 and U17 you have to qualify for this championship. Qualification goes by district of where the judoka lives. There are seven districts and each district gets the same amount off entries. Four judokas in each weight category qualify at the U15 category and two at the U17 category. Finalists from the year before, if still in the same weight category are automatically qualified. The system they fight with is so called double repechage which means that everyone gets repechage. So even if a judoka lost his/her first fight he/she can still get a third place. In the draw the judokas who won a district are separated, also the finalists from last year are separated and can only meet in a medal fight.

Research question

What is the impact of month of birth on being successful at the highest level of competition of youth judokas in the Netherlands? And sub questions: Does this impact get smaller at older age categories? Is there a difference in RAE between boys and girls in Dutch youth judo?

Hypotheses

The following hypotheses are based on the already existing literature on the RAE, which is discussed above. For the main question the hypothesis is that judokas born in the first quarter of the year are expected to have won more medals than later born judokas of the same year of birth. Furthermore there is expected to be more RAE in the U15 age category than in the U17 age category. And for the second sub question it is expected that there will be a smaller RAE for girls than for boys.

Data collection and Methodology

‘Successful’ will be measured by winning a medal at the highest level of national Dutch competition for that age category. The date of birth and gender from judoka who have won a medal at Dutch National Championships U15 years are obtained from judoinside.com for the years 2000 to 2012 (N = 886). The same has been done for judoka who won a medal at Dutch National Championships U 17 for 2000-2012 (N = 829). After 2012 the U17 age category was changed to under eighteen. Analysis on U15 and U17 is to see if differences get smaller at higher age.

Classify the months of birth to four quarters. January – March is quarter 1 (Q1), April – June quarter 2 (Q2), July - September quarter 3 (Q3) and October – December quarter 4 (Q4). Judoka with a missing date of birth were left out of the analysis. Further adjustments of the dataset can be found in Appendix 1.

Classify the relative age within the age category to compare judokas with the same age class. An age class is all the judokas that will become for example twelve years old in that calendar year.

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7 The age category U15 consists of three age classes together and for the U17 this is the same. 1 is oldest, 2 is middle and 3 are youngest within age category. The relative age within each age category is to prevent that judokas that won more than one medal to be counted twice in the analysis. Only for the age category U17 and relative age is equal to 3 some double names occur due to a change in rules during the investigated period. In the early years all U15 judokas might compete in the U17 age category, later they changed this to only the last year U15 judokas. All judokas that were U15 when they won a medal in the U17 age category got a relative age of 3.

The statistical program SPSS was used to do the calculations and the significance level is set at P < 0,05 which is commonly used to test RAE. A χ2 goodness of fit test was used to test whether the observed data significantly differ from the expected distribution of birth dates. The distribution of birth is assumed to be equally over the year, so every quarter 25%, similar to most studies on RAE. The χ2 goodness of fit test is done on all 3 relative age categories within both U15 and U17. The χ2 goodness of fit test is also performed on boys and girls separately, with no age category distinction and with age category distinction. This was done because some samples got too small when the age categories got separated. Some names occur double without the age category distinction because they won a medal in both age categories at the same relative age within that category.

The reliability of this method, with the 4 quarters and χ2 goodness of fit test, is proven in almost all previous studies on RAE. Therefor the outcome can also be compared with the outcome from Albuquerque et al. (2015) to a certain extent.

Data analysis

Table 1 represents the distribution of Dutch national judo championships medalists for the years 2000-2012 based on age category, no gender distinction was made here. The distribution of the relative age within the age category can be found in the first columns. Next columns show the quarterly distribution numerical and percentage wise for both age categories U15 and U17. In the column Qexp the expected amount of judokas born in every quarter, assumed 25 percent each quarter, is presented. The last columns present the outcome of the χ2 goodness of fit tests that where preformed. For the P value in the U17, relative age equal to 3, marked with *, some double names occur, see the methodology part for further explanation. The total sample within each age category is not tested, marked with **. This is because there were many double names, many judokas that won more than one medal in the three years they competed in that age category.

Table 2 represents the distribution of Dutch national judo championships medalists for the years 2000-2012 based on gender distinction, no age category distinction was made here. The distribution of the relative age within the gender can again be found in the first columns. Next columns show the quarterly distribution numerical and percentage wise for both genders. In the column Qexp the expected amount of judokas born in every quarter, assumed 25 percent each quarter, is presented. The last columns present the outcome of the χ2 goodness of fit tests that where preformed. In the samples doubles names can occur for judokas that won a medal in both age categories at the same relative age within the age category, marked by*. The total sample within each age category is not tested, marked with **. This is because there were many double names, many judokas that won more than one medal in the six years they competed, three years U15 and three years U17.

Table 3 represents the distribution of Dutch national judo championships medalists for the years 2000-2012 based on gender distinction and age category. Again the distribution of the relative age within the age category can be found in the first columns. Next columns show the quarterly distribution numerical and percentage wise for both age categories, U15 and U17 and gender each

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8 separately. In the column Qexp the expected amount of judokas born in every quarter, assumed 25 percent each quarter, is presented. The last columns present the outcome of the χ2 goodness of fit tests that where preformed. In the samples doubles names can occur for judokas that won more than one medal in that age category, marked by*. Two samples for boys, marked by **, could not be tested because the sample sizes, respectively N = 29 and N = 27, were too small.

Graph 1 shows the distribution of the relative ages within each age category for boys and girls separately. The percentage is on the Y-axis and relative age within the age category is on the X-axis. The relative age of 1 stands for oldest, 2 for middle and 3 for youngest judokas within age category.

Graph 1: Distribution of Dutch national judo championships medailist 2000-2012 by relative age within each age category

0,00% 10,00% 20,00% 30,00% 40,00% 50,00% 60,00% 70,00% 80,00% 1 2 3 P e rc e nt ag e

Relative Age within age category

Relative age by gender within each age category

Boys U15 Girls U15 Boys U17 Girls U17

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Table 1: Distribution of Dutch national judo championships medailist 2000-2012 based on age category, no gender distinction

AgeCategory RelativeAge N N% in AgeCat Q1 Q1 (%) Q2 Q2 (%) Q3 Q3 (%) Q4 Q4 (%) Qexp χ2 df P value

U15 1 576 65,01% ₁₉₁ _33,16% ₁₅₀ _26,04% ₁₃₁ _22,74% ₁₀₄ _18,06% _144,0 _27,875 ₃ _0,000 2 223 25,17% ₇₃ _32,74% ₅₄ _24,22% ₄₅ _20,18% ₅₁ _22,87% _55,8 _7,870 ₃ _0,049 3 87 9,82% ₂₇ _31,03% ₂₃ _26,44% ₁₅ _17,24% ₂₂ _25,29% _21,8 _3,437 ₃ _0,329 All together** 886 ₂₉₁ _32,84% ₂₂₇ _25,62% ₁₉₁ _21,56% ₁₇₇ _19,98% _221,5 U17 1 463 55,85% ₁₄₂ _30,67% ₁₂₄ _26,78% ₁₀₈ _23,33% ₈₉ _19,22% _115,8 _13,242 ₃ _0,004 2 246 29,67% ₈₀ _32,52% ₆₃ _25,61% ₅₂ _21,14% ₅₁ _20,73% _61,5 _8,862 ₃ _0,031 3 120 14,48% ₃₇ _30,83% ₂₃ _19,17% ₂₄ _20,00% ₁₉ _15,83% _25,8 _7,097 ₃ _0,069* All together** 829 ₂₅₉ _31,24% ₂₁₀ _25,33% ₁₈₄ _22,20% ₁₅₉ _19,18% _207,3

*some double names (see methodology for further explanation) **not tested, many double names in the sample

Table 2: Distribution of Dutch national judo championships medailist 2000-2012 based on gender, no age category distinction

Gender RelativeAge N N% in AgeCat Q1 Q1 (%) Q2 Q2 (%) Q3 Q3 (%) Q4 Q4 (%) Qexp χ2 df P value

Boy 1 614 69,54% ₂₂₁ _35,99% ₁₆₂ _26,38% ₁₄₁ _22,96% ₁₀₀ _16,29% _153,5 _41,674 ₃ _0,000* 2 213 24,12% ₇₉ _37,09% ₅₃ _24,88% ₃₆ _16,90% ₄₅ _21,13% _53,3 _19,319 ₃ _0,000* 3 56 6,34% ₂₇ _48,21% ₁₅ _26,79% ₆ _10,71% ₈ _14,29% _14,0 _19,286 ₃ _0,000* All together** 883 317 35,90% 230 26,05% 183 20,72% 153 17,33% 220,75 Girl 1 425 51,08% ₁₂₂ _28,71% ₁₁₂ _26,35% ₉₈ _23,06% ₉₃ _21,88% _106,3 _4,939 ₃ _0,176* 2 256 30,77% ₇₄ _28,91% ₆₄ _25,00% ₆₁ _23,83% ₅₇ _22,27% _64,0 _2,469 ₃ _0,481* 3 151 18,15% ₄₁ _27,15% ₃₃ _21,85% ₄₁ _27,15% ₃₆ _23,84% _37,8 _1,238 ₃ _0,744* All together** 832 237 28,49% 209 25,12% 200 24,04% 186 22,36% 208,0 *Double names can occur for judokas who won a medal in both age categories at the same relative age

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Table 3: Distribution of Dutch national judo championships medailist 2000-2012 based on gender and age category

Gender AgeCategoryRelativeAgeN N% in AgeCat Q1 Q1 (%) Q2 Q2 (%) Q3 Q3 (%) Q4 Q4 (%) Qexp χ2 df P value

Boy U15 1 328 72,89% 116 35,37% 85 25,91% 74 22,56% 53 16,16% 82,0 25,244 3 0,000 2 93 20,67% 33 35,48% 25 26,88% 13 13,98% 22 23,66% 23,3 8,806 3 0,032 3 29 6,44% 13 _44,83% 9 _31,03% 2 _6,90% 5 _17,24% _7,25 _** All together 450 162 36,00% 119 26,44% 89 19,78% 80 17,78% 112,5 36,453 3 0,000* U17 1 286 66,05% 95 33,22% 77 26,92% 67 23,43% 47 16,43% 71,5 16,825 3 0,001 2 120 27,71% 46 _38,33% 28 _23,33% 23 _19,17% 23 _19,17% _30,0 11,933 3 _0,008 3 27 6,24% 14 51,85% 6 22,22% 4 14,81% 3 11,11% 6,8 ** All together 433 155 _35,80% 111 _25,64% 94 _21,71% 73 _16,86% _108,3 33,614 3 _0,000* Girl U15 1 248 56,88% 75 30,24% 65 26,21% 57 22,98% 51 20,56% 62,0 5,226 3 0,156 2 130 29,82% 40 30,77% 29 22,31% 32 24,62% 29 22,31% 32,5 2,492 3 0,477 3 58 13,30% 14 _24,14% 14 _24,14% 13 _22,41% 17 _29,31% _14,5 0,621 3 _0,892 All together 436 129 29,59% 108 24,77% 102 23,39% 97 22,25% 109,0 5,450 3 0,142* U17 1 177 44,70% 47 26,55% 47 26,55% 41 23,16% 42 23,73% 44,3 0,695 3 0,874 2 126 31,82% 34 _26,98% 35 _27,78% 29 _23,02% 28 _22,22% _31,5 1,175 3 _0,759 3 93 23,48% 27 29,03% 19 20,43% 28 30,11% 19 20,43% 23,3 3,129 3 0,372 All together 396 108 27,27% 101 25,51% 98 24,75% 89 22,47% 99,0 1,879 3 0,598*

* double names can occur for judokas that won more than one medal in an age category ** sample size too small

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Conclusion

As expected there is a significant impact of month of birth on being successful at the highest level of competition of youth judokas, if divided into quarters, but only under certain circumstances. As shown in table 1 above RAE is proven for the relative ages 1 and 2 within the U15 age category (χ2 = 27,875; p < 0,001) and (χ2 = 7,870; p = 0,049) and within the U17 age category (χ2 = 13,242; p = 0,004) and (χ2 = 8,862; p = 0,031) where no distinction was made between gender. Within the same group RAE was not found in the relative age category 3 within both age categories, U15 (χ2 = 3,437; p = 0,329) and U17 (χ2 = 7,097; p = 0,069). An explanation for this can be that these samples where smaller respectively: N = 87 and N = 120 where all other samples were at least two times bigger.

Looking at table 3, the samples consist of unique names. So gender and age category

separated, a big difference for RAE between boys and girls was found. The hypothesis was that there would be a stronger RAE for boys than for girls. The results show that there is no RAE found at all for girls, not even for all girls within an age category together. For boys a strong RAE was found in all tested cases. The problem here was that some of the samples where relatively small. The sample for boys with relative age equal to 3 in both U15 and U17 age category even got too small to perform the χ2 goodness of fit test on.

To get bigger samples the age categories where merged. This leads to separation only by gender and relative age. The results in table 2 show again that there is a big difference between RAE for boys and girls. The results show that there is still no RAE at all for girls within the relative age categories 1(χ2 = 4,939; p = 0,176), 2 (χ2 = 2,469; p = 0,481) and (χ2 = 1,238; p = 0,744). For boys a strong RAE was found for within all 3 relative age categories (χ2 = 41,674; p < 0,001), (χ2 = 19,319; p < 0,001), (χ2 = 19,286; p < 0,001).

The last thing found in the data was that there was a difference between the percentage of medal winners for boys and girls within each relative age category, as shown in graph 1. Girls with a relative age of 3 within their age category have a bigger chance of winning a medal than boys in the same relative and real age category. On average boys have a higher relative age when they win a medal than girls. An explanation for this can be that judo is more competitive for boys than for girls. The difference between relative ages is smaller in the U17 category than in the U15 category both for boys and girls. This could be due to the assumption that RAE gets smaller at older age looking at physical development as mentioned by Musch & Grondin (2001). Further research is necessary to conclude this with certainty.

In summary it can be concluded that there is an impact of month of birth on being successful at the highest level of competition in Dutch youth judo. There is a difference between boys, where strong RAE was found in almost all cases, and girls, where RAE was not found at all. The difference between the age categories needs further research to conclude with certainty that it is decreasing at older age, but a decreasing trend for the relative age within the age categories was found. Finding RAE for boys medalists is in line with the study of Albuquerque et al. (2015) that found RAE for male Olympic medalists. In this thesis no tests were performed on different weight categories so on that matter no comparison can be made.

Review

Limitations to this study about RAE in the Dutch youth judo are that it is unknown which indirect personal factors affecting individual, the qualification system of the Judo Bond Nederland and the coaches/trainers. Personal factors affecting the individual are things as origin, income of the parents, family composition and psychological congenital aspect. Because judo is a sport with much physical contact some cultures prefer other sports to practice, especially for girls, almost all training groups are mixed which means they include both men and women that train together at all ages. Income of

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12 the parents is also an aspect that can limit a young judoka his development. Judo is not a cheap sport to practice if you also want to do competitions. All tournaments require a registration fee, thereby you need to drive there too because clubs only organize a limited amount of tournaments

themselves if they even organize tournament. Family composition includes the amount of brothers and/or sisters an individual has but also if his/her parents are divorced and at which age that happened. The more brothers and/or sisters an individual has the more the parents have to divide time between them and fit all the schedules together and try to prevent logistic problems. This can possibly mean that a child is not at the best training for him/her and therefore not developing

optimal. Domestically situations such as a divorce of parents or a loss of a relative can have an impact on the individual its mental state, such as concentration and motivation. The psychological

congenital aspect is the natural fighting spirit of an individual. There are individuals that cannot handle losses while some others do not even bother or even come out stronger because you can learn from your mistakes.

The qualification system of Judo Bond Nederland, where 4 judokas of each district qualify, can also be a limitation. It is not sure if you have the best judokas qualified. Some districts may have more and/or stronger judokas than others due to different amounts of population, population density and popularity of the sport in that region. A lower population density can mean that you have to travel further to find a judo club and therefore less people start judo. Also the more judokas you have on a club the more competition there is and the better you develop during practice games during training. A solution for this can be open registration, but the problem with that is that there will possibly be so much competitors willing to participate that there will be troubles with logistics.

Another factor is training and trainers. Not everybody has access to the highest level of training due to many possible circumstances, some of them mentioned above. Other circumstances can be that not all trainers have the same skills in teaching. It is also unknown if there are trainers that are already aware of the existence of RAE and educate and because of this knowledge select children in a different way. You need to have enough different partners to practice with. But again these partners also need to be of a certain level and a developed judoka has little to gain from a practice game with a beginner. Also a judoka of 20 kilo is normally no opposition for a judoka of 40 kilo.

Relevance of this research is to look into RAE in judo because if you make it till the highest international senior level you can live from the money you win combined with salary from the National Olympic Committee. This study is especially focused on the youth because RAE is already proven at Olympic level (Albuquerque, Franchini, Lage et al., 2015). It can also be relevant to see if there is RAE in Dutch youth judo to possibly make (more) people aware of its existence so it can be changed in the future. Trainers could alternate the method of educating judo to children and change the way most selections are made. This means not only selecting on being stronger at young age, but at the potential a young fighter has. Also the Dutch judo federation could change the way

competition for youth is set up in order to make chances more equal for individuals to perform at the highest level possible in the future.

Further research on RAE in Dutch (youth) judo could be done on weight classes, as already performed by Albuquerque et al. (2015) for Olympic athletes, where it was only found for heavy weights. Another point that can be looked into further is to make a better comparison between RAE in the U15 and U17 as already mentioned in the conclusion, since there was only found a downward trend in relative age within the age category compared to each other. A last option for further research could be looking into which judokas from the dataset really made it to senior level and is/was competing at international level.

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Appendix 1: Data Modifications

Year: 2001 Age Category: u15

According to the dataset there were seven price winners in the U50 weight category and only one in the U55. After looking at which weight class the judoka fought at tournaments with a data close by this event the following changes only to weight class were made because the position was correct. Dominique Moonen from U50 to U55

Tim Covena from U50 to U55 Maarten Lelsz from U50 to U55 Year: 2008 Age Category: u15

Two years of birth were impossible with the age category at that moment. Both were born 1991 according to the dataset, while the tournament was for judokas born 1994 to 1996. After further investigation the next changes are made.

Riff Ekelschot, weight category: U32, position: 3, date of birth removed from dataset. Anouk Samuels, weight category: U63, position: 1, 1991 changed into 1994