Statement of Purpose (SOP)

Hey, guys and girls. I was just wondering if any of the folks who have already been admitted to PhD programs could shed some light on the SOP. I'm not looking for detailed information on what you wrote in yours; rather, I'm just curious about how specific one needs to be regarding research interests. Hypothetically, could one just list an interest in Bayesian methods and internet marketing? Or must the interests be more defined, like "using Bayesian methods to explore how prior exposure to banner ads influence online consumer behavior" or something? I just made that up, but I think you know what I'm getting at here.

My assumption would be that the interests don't have to be very specific, because the odds are that the research topics investigated during the PhD probably won't be what we anticipated when we applied for admission. Even if we read scholarly articles now, we just don't know enough about the literature yet (in most cases) to know what areas need investigating. But on the other hand, I've been through the MBA process. When applying for programs, applicants and adcoms know full well that most people aren't sure exactly what they want to do and that many people change their minds during the program; however, applicants MUST be very specific in the essays.

Is the PhD SOP the same way? Do we need to be very certain about what we want to research, even if we all know full well that this will likely change? Do we need possible research questions, or just general topics like mentioned above?

Thanks!