The most pervasive mistake in thesis writing is treating the literature review as a summary of sources rather than a critical synthesis. Your examiner wants to see that you can analyse and position existing work — not just list it.
A weak methodology chapter is the second most common failure point. Every methodological decision must be justified — your choice of design, your sampling approach, your analysis framework. 'I used SPSS' is not a methodology.
Finally, many candidates fail to connect their discussion back to their original research questions. Your discussion section must close the loop explicitly: here is what I set out to find, here is what the data revealed, and here is what it means.
