Which of the following best describes a "measured variable"?

A "measured variable" is best described as a variable that is observed and recorded. This means that researchers collect data on this variable in its natural state, without changing or manipulating it. For example, in a study examining the relationship between sleep and cognitive performance, sleep duration would be a measured variable because researchers would observe how long participants sleep and then record that data for analysis.

The other options highlight different types of variables. A manipulated variable refers to a variable that is intentionally changed or controlled by the researcher to observe its effects on another variable. A constant variable is one that does not change throughout the study, which helps to ensure that any observed effects are due to the manipulated variables. An irrelevant variable would have no bearing on the outcomes or hypotheses of the study, thus it would not be a focus for measurement or analysis. These distinctions clarify the unique role that a measured variable plays in research methodology.