Your Big Marking Experiment ("Judgment calls", 18 September) notwithstanding, its comforting conclusion that academics across institutions mark similarly was, of course, flawed in one important respect.

There is only one person truly qualified to mark a piece of work. The teacher knows exactly what subject matter relevant to the assignment has been taught, tutored, emphasised or indicated from reading lists and in what depth.

Only the teacher knows what advice about written argument and correct use of sources has been given as well as what prior discussion or explanation of the assignment has taken place.

In short, the teacher is the only one who can tell whether or not the student has benefited from the tuition, or whether they could or should have known better.

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It is not just about outsiders making an abstract judgment about how well a question has been answered. They don't know enough about the context and there is every chance that, in the absence of that, they might reach different conclusions to teachers. The Times Higher Education experiment did not test this obvious point.

If universities trust academics to teach students, they ought to trust us to grade or moderate their work. Times Higher Education's experiment was based on the widespread, unresearched premise that there is a problem with marking, and thus markers, and on the demonstrably false assumption, for the reasons given above, that this can be resolved by external checking.

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Sam McKinstry, Professor of accounting and finance, University of the West of Scotland.