They don't even have to be overtly manipulated. They can be inadvertently manipulated by confirmation bias.

Researchers are supposed to put all of their effort trying to DISPROVE their assumptions. This is how the scientific method is supposed to work. Nowadays, this has been reversed in favor of: "my leftist professor told me ___ is true, so let me focus all my research on proving that it's true."

About a year ago, physicists created a picture of a black hole. These physicists understood how important confirmation bias is, so they explicitly followed a strict protocol where nobody could see the photo until all of the data was collected. They wanted to make sure that they wouldn't subtly change the methodology halfway through collecting data in favor of a certain preconceived assumption.

This is how research is supposed to be done. Sadly, almost nobody follows protocols like this.

They don't even have to be overtly manipulated. They can be inadvertently manipulated by confirmation bias.

Researchers are supposed to put all of their effort trying to DISPROVE their assumptions. This is how the scientific method is supposed to work. Nowadays, this has been reversed in favor of: "my leftist professor told me ___ is true, so let me focus all my research on proving that it's true."