1. Don’t write an essay on your subject.

This is a mistake that I made when writing my PS a couple of years back. I started philosophising on the deepest nature of my subject (History) repeating the point that History is very ambiguous and that historians have to grapple with a lack of objectivity.

A science applicant might write something along the lines of: ‘My subject of choice is very focused on problem-solving - I am interested in problem-solving - therefore I would be an apt student for the course.

The problem is that the academics who will be reading your PS know all these things and their subjects should not be explained to them.

It is OK to write a sentence or two on these things. However, after that, you will immediately need to switch the focus of your PS on yourself. Tell the readers why you would be a good applicant.

2. Don’t say controversial stuff

By this, I mean that you should not name-drop academically ambiguous stuff that would require a ton of space to discuss properly.

A politics PS I’ve read alleged that the EU and another country apparently had a clandestine agreement. Points like these are true eye-brow raisers. What the person said might have very well been valid but any controversial point necessitates a discussion—and discussions eat into the tiny amount of space you have.

3. Don’t name-drop books

This is a bit of a continuation to the previous point and especially relevant for humanities/social sciences applicants. For these subjects, one is typically expected to talk about extracurricular readings one has done. Thus, make sure that you only include books and articles that you think you’ve properly understood and will be willing to discuss in a possible interview.   

4. Don’t overcomplicate things.

Academics need to read tens if not hundreds of statements per each application cycle. Make sure that they understand what you are writing. Use short sentences and concise language.

Try to write a statement that you yourself would want to read.

Best of luck :)