I am applying for PhD programs at graduate schools in the US and I am wondering how I should refer to researchers that I name in my statement of purpose? In particular, I am wondering whether to refer to people that have titles of 'Assistant Professor' or 'Associate Professor' as Prof. or Dr.? I have heard that the prefix 'Prof.' tends to be reserved for full, tenured Professors - is that true?

(note, that I have edited the question due to its similarity to [this one](https://academia.stackexchange.com/q/37037/101), to focus specifically on the context of a statement of purpose for US grad schools)