A medicine to remove the redness in eyes was tested in a group of 100 students. Each student took either the medicine or a placebo in both eyes. The participants were given some free time after receiving their drops. The researcher did not keep track of what they did in that free time. It turns out that many of the people receiving the medicine spent their time outside and many of the people receiving the placebo spent their time inside. The results may now be biased because the variable location (inside vs. outside) is now a/an variable.

Independent variable

Explanation:

In a scientific experiment, an independent variable is that variable which the experimenter changes or manipulates in order to effect an outcome. Only one independent variable should be tested at a time in a good experiment, all other variables should be kept constant. The initial independent variable in this experiment is TYPE OF SUBSTANCE put in the students' eyes.

However, according to the question, the researcher did not keep track of what they did in that free time. Hence, many of the people receiving the medicine spent their time outside and many of the people receiving the placebo spent their time inside. The location of every student should be a CONSTANT VARIABLE i.e. same for all students but now it has become an INDEPENDENT VARIABLE because it has been changed.