Three of the best ways to collect data are through surveys, experimental studies, and observational studies. Each method has advantages and disadvantages, depending on the question we are trying to answer.

A statistical study begins with a research question, which clearly describes what we want to know. Most research questions are questions about characteristics of a population or about the effect of one variable on another.

Sometimes researchers have a question about a population, like: “What percentage of the fish in Lake Erie are toxic for humans to eat?” Sometimes researchers have a question about how two or more populations compare, like: “Which lake, Lake Erie or Lake Ontario, has a higher percentage of fish that are toxic for humans to eat?” And sometimes, researchers want to change one variable and see how a population responds. For example, “Does taking a fish oil supplement daily help older adults maintain brain function?” After the research question is created, the researcher needs to collect some data.

There are three methods for collecting data: observational studies, surveys, and experimental studies. In an experimental study, the researcher deliberately does something to one or more groups of individuals, such as giving them access to tutoring or giving them a vitamin, and then measures their responses in comparison to another group that does a different thing, such as not going to tutoring or taking a different type of vitamin. This is different from observational studies and surveys, in which the researcher collects data about individuals as they are.

In an observational study, the researcher records values for one or more variables, like ZIP code or height, for each individual participating in the study. These values can be obtained by observation, measure, or taken from existing data that has already been collected (like the U.S. census).

A researcher can also collect data with a survey, in which they ask each participant to answer one or more questions.

Each method comes with advantages and limitations that the researcher must understand before planning a study. The method used to collect data also affects what kind of conclusions can be drawn. Choosing the best method for a research question takes careful thought and practice.