Political polls taken before major elections are examples of research. case study survey experimental correlational

That would be survey research.
Surveys are used for the political polls as they are very good and appropriate for this kind of research as they consist of questions that have the aim of eliciting the specific information and data from the people about a particular subject.  Surveys can take a wide variety of forms be that a simple phone interview or a kind of online survey.

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.