The case, as best I can tell, for summative assessment as revealer of outcomes is this. Teachers teach, students learn then summative assessment takes place. It is diagnostic and while it may inform future practice it cannot, logically, change something that preceded it.
The assumption that assessment measures and does not change is built into our model for judging the performance of schools. We look at the grades children had on entry, the grades they got at the end and whether their families are below a certain poverty threshold. In this view Education is an operation performed on children, by schools. Children are of a measurable quality and their results show how well or badly this raw material was processed.
When you think about school on a day to day basis though the idea of students as the inert objects of teaching does not reflect reality. Students have agency and even if their choices are limited by strict behaviour policies to collaborating with their teachers and learning what they can, or resisting attempts to teach them and refusing to learn, they always have at least those two choices. A satisfactory answer to the question of whether outcomes are affected by assessment needs to account for why students might choose the latter.
To do this let's examine the experience of school from a child's perspective. Say there was a boy called Ricky, whose parents are not neglectful or abusive but neither are they bookish or preoccupied with toddler Ricky's language acquisition. Aged seven, something Ricky has long suspected is confirmed, his level, his attainment, is below the expected standard, below that of his classmates. This experience will be repeated again and again, with increasing frequency as Ricky’s education continues. Does that one-note tune, you’re worse than the other children Ricky, not affect Ricky’s motivation to try his best?
Perhaps it does but it shouldn’t. Ricky shouldn’t compare himself to others, he should focus on his own learning. After all, most tennis players are better than me but I don’t let that stop me playing tennis. The problem with this argument is that my income and social status are not dependent on my tennis ability, whereas they are intimately bound up with my academic performance as a child. We remind kids of this all time, we tell them you need good grades so you can go to university and get a good a job, but though we always state the case in the positive, the negative is just as clearly true, if you get bad grades you won’t go to university and you’ll get a bad job. Asking Ricky, a sophisticated primate but a primate nonetheless, not to care about his low position in our social hierarchy is like asking him to not be human.
The people who make decisions about Education come from all kinds of backgrounds but they invariably share the fact that they themselves did well at school. It’s easier for them to relate to the children who are succeeding as they as they succeeded and are positive about school as they were. Add in the fact that the children told they are failures are often disobedient and rude and the empathy gap grows. The former group, whose grades prove to the world they are the best, are motivated and work hard; the latter, whose grades prove they are the worst, are disaffected and defiant. That’s not a coincidence.