Final answer:
A descriptive study examines one variable, a correlational study examines the relationship between two measured variables, and a controlled experiment manipulates one variable to observe its effect on another, allowing for causal claims.
Step-by-step explanation:
A descriptive study reports information on one variable, whereas a correlational study typically looks at two variables that are measured rather than manipulated. A controlled experiment includes two variables, one of which is manipulated, and it is the only type of study that can make a causal claim.
In the context of scientific research, a descriptive study might involve the detailed observation and reporting of behaviors or characteristics without relating them to other variables. A correlational study, on the other hand, examines the correlation relationship between two variables to understand how they may move or change in relation to each other, determining the correlation coefficient.
A controlled experiment is an investigation in which researchers manipulate one variable (the independent variable) and observe the effect on another variable (the dependent variable), while also using a control group to minimize the impact of outside variables, thus allowing them to assert causal relationships.