tunalegs
Pretended Artist
I think Robert Frank had a bias in one of the really great photo books after WWII "The Americans" and I also think that most of the FSA photographers had a bias to show how hard the times were on many Americans and I know W.Eugene Smith had a bias in his photographs of Minamata the fishing village being poisoned by a Chisso factory dumping heavy metals into water sources. So sometimes a bias is a good thing.
Confirmation bias is the disregard for facts which conflict with your ideas about a subject. For example if you think a problem was caused by sexism - you will seek out facts which confirm this, but ignore facts which contradict this.
One can be biased but still acknowledge the existence of facts which do not support their conclusion - and wonder of wonders - document them. I don't really think that is too complicated to wrap one's head around.
I'm not claiming that it is possible for a single person to present a situation entirely 100% objectively. That is of course impossible. I'm just suggesting that if you claim to be documenting a subject, but you're cutting out everything that doesn't shore up your argument, what you're doing is more akin to making an advertisement for a product, than a documentary. It's like if you were attempting a "science experiment" with no controls.