Bias in an experiment can also be controlled by conducting blind experiments. In blind experiments, critical information is kept secret from the participants in order to guard against conscious and/or subconscious bias. There are two types of blind experiments: single-blind and double-blind. In single-blind experiments, critical information is kept from the test subjects, but the experimenter knows everything. In our aspirin example above, a single-blind experiment could be conducted by not revealing to the test subjects whether they are taking sugar pills or aspirin. This would prevent a scenario where people who took aspirin reported feeling better merely because they expected aspirin, but not sugar pills, to help them. The psychological expectation bias is erased if they don't know which type of pill they took. In double-blind experiments, both the test subjects and the experimenters are unaware of critical information. The information is only revealed to the experimenter once the data has been collected. For example, if neither the test subjects, nor the experimenters interviewing the test subjects, knew who had been given aspirin and who had been given sugar pills, it would be a double-blind experiment. The advantage is that the experimenters, because they were ignorant about the independent variable, would be unable to bias the test subject's answers by doing subconscious things like questioning the subjects who were given aspirin more vigorously than the subjects who were given sugar pills. Because they guard against a wider range of potential biases, double-blind experiments are considered the most scientifically rigorous; however, they can be challenging to implement as they require additional help by a third party who holds the key to which group—control or experimental—each test subject belongs.