Students in tertiary education are often faced with the prospect of writing an essay on a topic they know nothing about in advance. In distance learning institutions, essays are a common method of assessment in the UK, and specified course texts remain the main sources of information the students have. How do students use a source text to construct an essay? The present paper presents a methodology for mapping the source text on to the finished student essay. The underlying assumption is that students are using a form of imitative problem solving when faced with the complex task of writing an essay. Twenty-two essays written by Open University students in the UK, based on three different questions, were analysed on the basis of the order in which novel concepts were introduced and the extent to which this order mirrored that of the source textbook. Correlations were then carried out between the structure of the essay, the structure of the source text and the eventual grade awarded. The average correlation for all three essays and source texts was 0.8, with some individual essays having a correlation of 0.98, demonstrating that the students were closely imitating the argument structure of the source text.