When you form an opinion before you start reading on a subject, the reading process is altered. You start looking for evidence that supports your opinion, while dismissing information that goes against it, instead of giving all evidence equal consideration. Forming an opinion beforehand will also make you less likely to change your point of view as you gather more information, even in cases that are indefensible or wrong. If you care about the value of your opinions and about making informed choices, it is better to evaluate at the end instead of forming your opinion beforehand.