Does more instruction in primary school improve secondary school outcomes? a re-evaluation of recent work

Recent work in New Zealand predicts that students who receive an extra year of primary school instruction are 41 percent more likely to achieve NCEA Level 3. However, a simple comparison of achievement across students with the greatest average difference in instruction time shows no perceptible differences in outcomes. We explain this apparent paradox by demonstrating that improper sample creation in existing research has generated a spurious relationship. In particular, prior research included students from cohorts which were still progressing through secondary schooling. The consequence of this is that large temporary gaps in achievement are interpreted as long-run differences in outcomes. Correcting the sample yields results which indicate no significant impact of instruction time on secondary outcomes.