Final answer:
The true statement is that a single-blind experiment helps reduce researcher bias. A single-blind study prevents participant expectations from influencing results, while a double-blind experiment additionally helps reduce experimenter bias, making it typically more rigorous than a single-blind experiment.
Step-by-step explanation:
The true statement is: a. A single-blind experiment helps reduce researcher bias. In a single-blind study, participants do not know whether they are receiving the treatment or a placebo, but the researchers do. This setup helps to prevent the participants' expectations from affecting the results. However, since the researchers are aware of who has been assigned to each group, there may still be some experimenter bias.
On the other hand, a double-blind experiment is designed to be more rigorous. Both the participants and the researchers are unaware of the group assignments. This method controls for both experimenter and participant expectations, reducing the risk of bias. The presence of a placebo and the assignment to control or treatment groups are examples of experimental controls, which are used to compare between groups that are different in only one independent variable.
Therefore, the statements b, c, and d are not universally true. While a double-blind experiment generally provides more accurate results due to minimized bias, it does not control all variables, nor does it eliminate all unknown variables, nor is it 'always' more accurate because there could be exceptions based on the specific context of the study.