Despite profound influence of selection-by-ability on children's educational opportunities, empirical evidence for the validity of 11-plus tests is scarce. This study focused on secondary selection in Kent, the largest grammar school area in England. We analysed scores from the 'Kent Test' (the 11-plus test used in Kent), Cognitive Assessment Tests (CAT4) and Key Stage 2 Standardised Assessment Tests (KS2) using longitudinal data of two year cohorts (N[subscript 1] = 95, N[subscript 2] = 99) from one primary school. All the assessment batteries provided highly overlapping information, with the decisive effect of content area (e.g. verbal vs maths) over task type (e.g. knowledge-loaded vs knowledge-free). Thus, the value in differentiating 'pure' (i.e. knowledge-free) ability in 11-plus testing is questionable. KS2 and Kent Test aggregated scores overlapped very strongly, sharing nearly 80% of variance; moreover, KS2-based eligibility decisions had higher sensitivity than the Kent Test in predicting the actual admissions to grammar schools after Head Teacher Assessment (HTA) appeals have taken place. Finally, the use of multiple pass marks for each Kent Test component, as well as the total score, was found to increase the chance of false rejection. This study provides preliminary evidence that national examinations could be a good basis for selection to grammar schools; it challenges the use of complex admission rules and multiple decisions, and questions the value of 11-plus tests.