In a sense, every post on this site seeks to answer the question of why polls seem contradictory. A good place to start is with the following three posts on the random variation inherent in all sample surveys and what a consumer can do to minimize it (each post has a link at the end connecting to the next post in the series):
Beyond that, much of the variation in political polls comes from the way the pollster defines a “likely voter.” These two questions on the FAQ list deal with two issues that explain much of the systematic variation between polls
Finally, at the most basic level, political campaign polls typically seek to accomplish three tasks. Click on any of these links to display short excerpts of all of the posts in the category in reverse chronological order (most recent first):
