It is the assumptions with which you fill in the gaps between the data sets, observations, tests and facts.
Your bias directly influences how you interpret data. Biases are never intellectual but emotional, which is why they are so hard to purge: they are always—100% of the time—tied to some species of trauma: a betrayal, a personal injury, an imagined sleight, an inaccurate presumption, actual or anticipated ridicule, etc.
If you do not believe red triangles exist…
you know, because you were beat up by a guy wearing a shirt on which was printed a red triangle, or because a giant plastic red triangle fell on you as a child and broke your leg, or because your mother abandoned you and your father by running off with a suave, stylish, European red triangle…
and I show you a picture of a red triangle…
you will simply smirk, then calmly and condescendingly unbraid me, and accuse me of being superstitious because I believe in red triangles. You will then smugly inform me that what is in the picture is not some mythical red triangle.
You will deign to reveal red triangles are mere pseudo-religious icons imagined by our savage, fire-adoring ancestors for arcane rituals to protect them from dangerous forces of nature they didn’t comprehend because they were ignorant cave-dwellers.
You will go on to explain the red triangle is in fact, an illusion, a misunderstanding (on my part, of course) of geometric shapes and color theory.
You will end your rebuke by explaining my continued belief in red triangles is little more than a religious dogma, and that I am ultimately delusional. In fact, all the tens of billions of people who ever believed or presently believe in red triangles are also delusional. You and your tiny cult of deniers alone see through the daydream that has so easily ensnared the Human Race since time immemorial.
I would nod patiently, and when you are done, I would ask: ‘What is it then?’
You will look down your nose at me, smile, and patronize: ‘Well, it’s quite obvious. Any fool can see! It is a not-quite-orange three-sided square.’
World views come first.
Anyone who tells you differently is a barefaced liar.