<p>Each year in Australia, all students in Grades 3, 5, 7 and 9 undertake standardized literacy and numeracy assessments. One of the key purposes of this assessment program, the National Assessment Program: Literacy and Numeracy (NAPLAN), is to provide information about students’ attainment and growth in basic skills to schools and educational systems. The work in this thesis therefore applies a developmental perspective to the examination of achievement and growth in these basic skills. Two complementary approaches are used throughout to examine achievement across the span of NAPLAN assessments. First cross-sectional analyses, repeated at each assessed grade, provide insights into predictors of achievement, and how their influence may vary over time. Second, longitudinal methods examine growth trajectories of the reading comprehension and numeracy domains from Grade 3 through to Grade 9. The central aim of the work overall is to further understandings of the variation in achievement both at each assessment point and in trajectories of skills over time.</p> <p>The first chapter describes the background to the study which provided the data used in Chapters 2 to 4 of the thesis. The Academic Development Study of Australian Twins recruited 2762 Australian twin pairs, and 1465 siblings, and tracked them from Grade 3 through to Grade 9. A range of demographic and behavioural data was collected concurrent to the participants’ completion of NAPLAN tests. Chapters 2, 3 and 4 used data from one randomly selected twin and/or sibling to examine predictors of development in literacy and numeracy.</p> <p>The second chapter used cross-sectional methods to examine whether delaying the initial school start of students for one year was associated with higher attainment in NAPLAN assessments at each grade. Results of these analyses indicate that while old-for-grade students appear to have a slight advantage over their younger peers in Grade 3, this advantage fades out and is no longer evident in Grades 7 and 9. Furthermore, exploratory analyses examining inattention and hyperactivity suggest that these child-specific behavioural characteristics attenuate the observed early relations between delayed entry and achievement.</p> <p>The third chapter employed both cross-sectional, and longitudinal methods to examine whether students who attended private schools had systematically higher achievement on NAPLAN tests compared with their peers in public schools. Cross-sectional results at the four grades indicated that once the socioeconomic background of students was controlled, any associations between private school attendance and higher achievement largely disappeared. Further, latent growth curve analyses enabled comparisons of growth for groups of students attending different school sectors at the four NAPLAN grades. Results showed that growth trajectories in reading comprehension and numeracy were no different for those attending public schools, those attending private schools, and those changing from public to private schools as they entered secondary school in Grade 7.</p> <p>The fourth chapter focused more specifically on the development of reading comprehension, examining how growth in this academic skill is related to developmental change in inattention from age 8 through to 14. This study compared two latent growth modelling approaches: a multivariate latent growth curve model allowing intercepts and slopes of the two domains to correlate, and an autoregressive latent growth model with structured residuals, which separates out between-person relations from within-person growth. Results demonstrated that while reading and inattention were strongly related in Grade 3, relations between the developmental trajectories of these two domains were inconsistent.</p> <p>The final chapter examined whether Matthew Effects were evident in longitudinal population data sourced from two Australian states, NSW (<i>N</i>= 88,958) and Victoria (<i>N</i>= 65,984). Again, latent growth curve analyses were employed to examine variation in growth trajectories in NAPLAN reading comprehension and numeracy tests at the four grades. Results demonstrated that, rather than Matthew Effects, growth in both domains in both states was characterised by compensatory growth patterns. These patterns suggest that students who start with below average attainment at Grade 3 make more progress in their basic skills attainment over time, compared with their peers who start with above average achievement.</p> <p>Replicating work from the smaller samples of Chapters 2 to 4 with population-level data provides information about development across the full distribution of achievement, and additional evidence that results in the first four chapters are generalisable to the population</p> <p>The work presented in this thesis contributes to an understanding of how literacy and numeracy skills develop from middle childhood through to adolescence. Results have the potential to inform future targets for educational policy revision (Chapters 2 and 3), the timing of academic interventions (Chapter 4), and provide baseline information about growth trajectories in reading and numeracy (Chapter 5) that can be useful in evaluating school- or system-level changes to educational practices in Australia.</p>