Final answer:
The dosage of BPA administered is an independent variable, which researchers manipulate to observe effects on a dependent variable.
Step-by-step explanation:
The dosage of BPA administered is a independent variable. In experiments, the independent variable is the factor that is changed or manipulated by researchers to observe its effect on a dependent variable. For example, in the LibreTexts scenario, if researchers are investigating the effects of BPA on the expression levels in offspring, the dosage of BPA would be the independent variable, and the expression levels would be the dependent variable. This can be compared to studies like Salk's polio vaccine trial, where the vaccine was the independent variable and the incidence of polio was the dependent variable. In a pharmaceutical study described, where a new drug is administered to lower blood pressure, the independent variable is the presence of the drug while the dependent variable is the blood pressure levels of the participants. Differences between the control group receiving a placebo and the treatment group can produce variability in results attributable to the independent variable's effect.