Flanders has a history of participating in International Large-Scale Assessment studies (ILSAs) like TIMSS, where it has often ranked highly. However, the last cycles of TIMSS have shown a gradual decline in the academic achievement of Flemish students. This has sparked a debate about the quality of education in Flanders. Between the TIMSS cycles of 2015 and 2019, Flemish students' achievement levels decreased by 14 points for mathematics and 11 points for science (Faddar et al., 2020).

Although ILSAs are crucial tools for policymakers to assess the quality of educational systems, their primary purpose is periodic benchmarking (Addey and Sellar, 2019). However, the decline found among Flemish students has prompted a deeper investigation and monitoring of the evolution of Flemish learning gains throughout the remaining two years of primary schooling, which goes beyond benchmarking. To this end, a longitudinal study based on TIMSS-2019 was set up in Flanders: TIMSS-repeat.

Using a longitudinal design, TIMSS-repeat retested students who participated in the TIMSS 2019 cycle in 2021, when most of the students were in the sixth grade of primary education. In total, 4.301 students, their teachers, and their school principals participated in TIMSS repeat. The main purpose of TIMSS-repeat was to investigate the learning gains of Flemish students during the last two years of primary school, allowing an inquiry into the connection between students' background characteristics and their learning gains for mathematics and science. Moreover, the specific timing of the data collection in May 2021, just after the school closures and quarantines due to COVID-19, allowed for additional information regarding the impact of COVID-19 to be collected. This enabled the investigation of COVID-19's impact on the learning gains in mathematics and science. The following research questions were central:

• What are the achievement gains for Flemish students in the last two years of primary education?
• What are the differential effects of student background characteristics on their achievement gains in the last two years of primary education?
• How did the COVID-19 pandemic and the resulting school closures impact student achievement gains in Flanders?

The first and second questions aim to analyze how students in Flanders progress through the last years of primary school. With these questions, we aim to reveal how students’ learning gains increase and whether specific background characteristics facilitate or hamper student learning gains. In previous TIMSS studies, it was found that home language or students’ socioeconomic status is linked to their achievement (Faddar et al., 2020; IEA, 2020). The third question seeks to provide valuable information to both researchers and policymakers regarding the impact of the COVID-19 pandemic on student learning and achievement. Not only does TIMSS-repeat in Flanders provide answers to these research questions, but it also aligns with the research goals of the TIMSS longitudinal study that is following the TIMSS 2023 cycle (The International Association for the Evaluation of Educational Achievement (IEA), 2022).

TIMSS-repeat in Flanders provided a valuable but tentative insight into Flemish learning gains during the final grades of primary education in Flanders, characterized by one of the most impactful global events of our time. In this presentation, we will discuss the different steps taken to conduct the TIMSS-repeat study in Flanders and present our most important findings.

The research presented here utilizes data from the TIMSS 2019 cycle collected in May 2019 (T1) and a repeated measurement after two years in May 2021 (T2), based on the same sample of schools and students. For T2 91.9% of the schools from TIMSS 2019 agreed to participate, resulting in a sample of 4301 students nested in 133 schools. Rigorous checks were conducted for selection bias in comparison to the T1 sample, including factors from both the school and the student level such as school performance, educational network, geographical location, gender, and socioeconomic status. Both T1 and T2 samples are comparable, revealing no significant selection bias, and this is on both the school and the student level. To ensure the reliability of the data, several precautions were taken. To avoid a modus effect (Martin et al., 2020), paper-based achievement tests were administered for both T1 and T2. Additionally, to minimize the likelihood of a ceiling effect, adjustments were made to the test materials: easier items were excluded and more difficult mathematics and science items were included from the Flemish national assessment tests conducted in 2015, 2016, and 2021. In the selection of these new items, we maintained a distribution that aligns with the content domains (measurement & geometry, numbers and data for mathematics; life, physical, and earth for science) and cognitive domains (knowing, applying, and reasoning) (Martin et al., 2020). To allow for a precise description of the learning gains, the test items of T1 and T2 were calibrated (Scharfen et al., 2018). Finally, to avoid a retest effect individual students were administered different test items compared to the 2019 test. To grasp the impact of COVID-19, specific scales were added to the background questionnaires for the students, teachers, and school leaders. All new instruments were found to be reliable and valid. The analysis began by calculating weights, jackknife estimates, and plausible values for students’ mathematics and science achievement (Martin et al., 2020). The R package “EdSurvey” was used for all analyses (Bailey, 2020), specifically the “mixed.sdf” function was used to estimate mixed effect models mapping differential effects of student characteristics on achievement. The analysis used a scale ranging from 0 to 1000 points.

Looking at the first research question, Flemish pupils demonstrated achievement gains in both mathematics and sciences over the last two years of primary education, with an increase of 117 points in mathematics and 107 points in science. In terms of cognitive domains, Flemish students exhibited the most significant improvements in the Applying domain for both mathematics and science, aligning with Faddar's hypothesis regarding the emphasis on higher cognitive skills in later years of primary education (Faddar et al., 2020). Answering the second research question, we found that boys obtained slightly higher learning gains compared to girls, with an increase of 120 points in mathematics and 113 points in science, compared to 116 and 109 points, respectively. For home language, noteworthy results were found: students who never spoke the language of the test at home demonstrated the most substantial achievement gains in both mathematics (137 points) and science (134 points). Additionally, students with a room for themselves and access to a significant number of books at home experienced the highest achievement gains in both subjects. When answering the first and second research questions, caution is advised: while we found learning gains, empirical evidence to compare the size of these learning gains is lacking. Potential benchmarks such as Bloom et al. (2008), Martin et al. (1997), or Mullis et al. (1997) are based on empirical data, but may also not be as pertinent due to their age and dissimilar contexts. Finally, the descriptive data on how schools, teachers, and students adapted to COVID-19 provides an answer to the third research question.Results include, among others, a shift in didactics and teaching and difficulties with online teaching.

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