What is public opinion, and why is so much time and effort spent on measuring it in the united states? what techniques are used to obtain an accurate reading of public opinion? are public opinion poll results generally accurate? explain?

Public Oponion is used to measure the popularity of a president for example. They take say 1/50 people and ask them their oponion. They cant go all over the united states and ask people so they take a sample population. They arent completely accurate due to this.