Which of the following best describes an independent variable?

An independent variable is defined as a presumed cause that influences or affects the dependent variable in an experiment. It is the variable that researchers manipulate or alter to observe the effect it has on another variable, which is the dependent variable. By changing the independent variable, researchers can determine if it leads to a change in the dependent variable, thereby allowing for insights into causal relationships.

In contrast, other definitions do not accurately reflect the role of an independent variable. For instance, a variable that is controlled in an experiment refers more to constants or controlled variables, which are kept stable to prevent them from affecting the outcome. A variable that is measured as an outcome describes the dependent variable itself, not the independent variable. Lastly, a fixed variable that does not change during research is typically associated with controlled variables or constants rather than independent ones, which are intentionally manipulated. Therefore, the option that highlights the independent variable as a presumed cause is the most accurate description.