I am applying for graduate programs in Physics (at the US universities) for Fall 2018. I am writing my Statement of Purpose and I read a few successful SoPs before writing my own and it made me all confused. All of them talked all about what they had done and had only a few sentences here and there talking about what they liked or what was their purpose. In the end, they talked about what topics or research groups they would like to work with using just one paragraph. It is safe to say that more than 90% of each of the SoPs I have read talked about what they have done rather than talking about their purpose. So, I want to confirm whether it is the custom or they were not really good SoPs? I understand that stating only the purpose does no good and one must back it up by showing a corresponding background but I think a ratio of something like 60% background 40% purpose is the maximum I would like to lean towards the background. Is this a wise thing to do?