What method can effectively reduce investigator effects in a study?

Using a double-blind procedure is an effective method for reducing investigator effects in a study. This approach ensures that neither the participants nor the researchers interacting with them have knowledge of key aspects of the study, such as which group participants belong to (experimental or control). By preventing both parties from knowing these details, the potential for bias in data collection, participant behavior, and interpretation of results diminishes significantly.

For example, if a researcher knows that a participant is in the control group, they may unintentionally influence the participant's responses or behavior, consciously or unconsciously. This can lead to skewed results that reflect the researcher's expectations rather than the actual effects of the independent variable being studied. A double-blind design mitigates this risk, allowing for a more objective and reliable collection of data.

Other methods such as conducting research alone or simply relying on experienced researchers do not adequately address the potential biases that can arise when the researcher is aware of group allocations or the specific hypotheses being tested. Increasing participant numbers might enhance statistical power but does not fundamentally alleviate the risk of investigator effects from influencing the outcomes. Thus, adopting a double-blind procedure is the most effective technique among the alternatives provided for minimizing such biases.