Quantitative research methods deal with measurable and numerical data to systematically investigate phenomena and relationships. There are several types of quantitative research, including descriptive research which identifies characteristics of phenomena and explores correlations, observational studies which use rating scales and multiple raters to define and observe behaviors, and correlational research which examines differences between variables of entities and looks for relationships where one variable changes with another. Survey research involves studying samples from populations to discover trends, distributions, and interrelations.