Diferentiation of initial skills in 1 st grade and the relative age efect in 3 rd grade Krzysztof Konarzewski Educational Research Institute* Te paper aims to verify two hypotheses. Te frst states that the diferentiation of initial 1 st grade pupil skills is negatively correlated with age at entry to school. Tis was tested according to fndings from the ERI study Six and seven year olds at the start of school. Te variation of reading and writing scores of 1164 pupils aged 5.9–7.9 years who were beginning 1 st grade decreased in subsequent age quartiles. Te second hypothesis – that classroom diferentiation of pupils’ initial skills in 1 st grade is positively correla- ted with the birthdate efect in 3 rd grade – was tested on data of 4838 pupils from 254 classrooms drawn from the 2011 PIRLS and TIMSS studies in Poland. Skills were evaluated on the basis of parental reports. Hierarchical linear analysis (gender, SES and school location controlled) showed that the greater the dif- ferentiation of language skills in 1 st grade, the greater the birthdate efect in mathematics in 3 rd grade. Tis result suggests that school entry age is of lesser importance than the methods used to reduce diferences in children’s school readiness at the onset of education. Keywords: education; school readiness; birthdate efect; PIRLS; TIMSS; HLM. © Educational Research Institute Edukacja 2017, 2(141), 100–112 doi: 10.24131/3724.170206 issn: 0239-6858 * Address: ul. Górczewska 8, 01-180 Warszawa, Poland. E-mail: konarzewski@neostrada.pl it is indistinguishable from zero. The differ- ences in achievement of girls and boys tend to diminish i n a similar way. The average age of pupils in the class depends on legal regulations and social standards. For this reason, the interaction can be detected only in international data. Any explanation of this interaction must be embedded in the time perspec- tive. Many studies show that the difference between achievement of older and younger pupils in a class (referred to as the relative age effect) decreases in consecutive years of education (DiPasquale, Moule and Fle- welling, 1980; Dolata and Pokropek, 2012; Hutchison and Sharp, 1999, after: Sharp, 2002; Jones and Mandeville, 1990; Lan- ger, Kalk and Searls, 1984; Verachtert, De T he aim of this article is to explain an intriguing phenomenon: a small but statistically significant part of scholastic achievement of ten-year-old pupils from 25 European countries is dependent on the interaction of the relative and absolute age of pupils (Konarzewski, 2013). The interaction is illustrated in Figure 1. Individual points represent average mathematics achievement of children divided into groups by three cri- teria: average age at school entry (6.2, 6.6, and 7.1 years), relative age in the class (younger, middle-aged, older) and gender. We can see that the achievement of the oldest pupils in their classes is higher than the youngest pupils, and this difference is the greatest in early-start classes, while in late-start classes,