Final answer:
To improve internal validity and reduce error variance, participants are put through all conditions of the IV, with random assignment evening out lurking variables and establishing causality.
Step-by-step explanation:
Putting every participant through every level of the independent variable (IV) in a research study serves to reduce error variance and improve internal validity. When all participants are exposed to each condition of the IV, the effects of potential lurking variables are minimized because these variables are evenly distributed across all conditions. This design ensures that any observed effects on the dependent variable are likely due to the manipulation of the IV rather than to other factors. Random assignment of participants to different levels or orders of the IV plays a crucial role. It ensures that the groups are equivalent at the start of the experiment, thereby eliminating the influence of potential lurking variables. This enhances the causality of the study, providing stronger evidence that changes in the IV are responsible for any observed differences in the response variable. In conclusion, by having all subjects experience every level of the IV and randomly assigning the order of treatments, the study controls for differences other than those introduced by the researcher. This approach is central to a well-designed study and is essential in establishing a cause-and-effect relationship between the explanatory (IV) and response variables.