There are several kinds of pre-approvals. Most loan officers these days have access to an automated underwriting program on their laptop computers that gives them the authority to run your credit and render an automated credit decision. It can be done pretty quickly. The information that the loan officer enters into their system to get the approval is not ironclad, however. For example, a borrower can tell me over the phone that they make $5,000 a month, but until I see the pay stubs and supporting W-2 forms that support it, I am only taking the borrower's word for how much they really make. So, in reality, the strongest form of pre-approval would be once the borrower has submitted income and asset documentation and a live, breathing underwriter has reviewed the documentation and rendered an approval behind the automated preliminary decision. Even the most seasoned loan officer can make a mistake in the pre-qualification process these days with all of the changes that have come about lately....The best pre-approval is always going to be that second pair of eyes on the pre-approval process....Hope that helps.