The more information you ask users, the less likely they are to complete a process. But it's not the amount that matters most.

It's how much sense it makes for the user to give certain information.

Users don't see any benefits to answer «Where did you hear about us?». That's why this information is usually not reliable. Users give sloppy answers, because their answers don't affect them.

Take this action

Make sure, to only ask for information the user sees a need or a benefit for.