I am an Indian citizen living in the US and a keen follower of the US Politics. I would like to understand one concept. Whenever there was a survey, it inherently included a democratic tilt ( which was true ) or a republican tilt ( which was wrong). In fact, in their final survey, I believe Pew survey include D +6.
How and what are these tilts / biases based on? Are these subjective? Please help me understand.