In statistical modeling, an explanatory variable is what a researcher manipulates or observes changes in, while a response variable is what changes as a result. The response variable is also known as the dependent variable, and it depends on another factor, known as the independent variable, to cause change or response to it. The term "response variable" is preferred by some authors over "dependent variable" when the quantities treated as independent variables may not be statistically independent or independently manipulable by the researcher. The response variable is the subject of change within an experiment, often as a result of differences in the explanatory variables. In other words, the response variable is the expected effect, and it responds to explanatory variables. For example, in a study comparing two cancer treatment approaches, chemo or surgery, the response variable is efficacy in the form of survival time, while the two different approaches define the explanatory variables.