Do you put a header on the personal statement and diversity statement differentiating between the two?

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i did. just "Personal Statement".

PS or DS as the title, not in the header as well?

are there standard guidelines somewhere online?

I totally understand the paranoia of making sure everything looks right for something so important, but at the same time, you're applying to grad school, and so I think most of them are going to assume you can format a paper in a presentable way. There aren't "standard guidelines;" just read the directions. Some schools will tell you to label the statement with the question number from the application, but most just tell you to go for it and give you a suggested length. Make it presentable, clear-cut, organized, and you're set.

That said, in my headers, I put my name top left, the title (which most of the time was ____(school name) Personal Statement), and "LSAC #______" top right.

Ditto.

I think this seems to be the way most people go as well.

I just put down my name and LSAC number without specifying my PS and DS. I hope they don't assume it's a 3.5-page PS

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