Bias in Experiments: Placebo & Blinding#
Even with randomization and careful design, participants can change their behavior simply because they know they are being studied. This can introduce bias into an experiment.
Placebo Effect#
The placebo effect happens when people show improvement even though they received a “fake” or placebo treatment with no active ingredient. The improvement comes from expectations, not the treatment itself.
Example: In medical trials, patients receiving a sugar pill may still report less pain because they believe they are receiving medication.
Figure 8. Illustration of the placebo effect showing how participants’ expectations can influence outcomes even without an active treatment. Source: sciencenotes.org
The placebo effect is a form of expectation bias. To minimize it, experiments use placebo controls so that both groups have similar expectations. This helps separate the true treatment effect from psychological or expectation effects.
How to Minimize Bias? Blinding (Single-Blind / Double-Blind)#
Blinding means that participants, researchers, or both do not know who received the treatment. This prevents behavior or measurement from being influenced by expectations.
Single-blind: participants do not know which group they are in.
Double-blind: neither participants nor researchers know, until the experiment ends.
Figure 9. Single-blind vs. double-blind study designs. Source: sciencenotes.org
Blinding, especially double-blinding, is a common method for minimizing expectation bias and making experimental results more trustworthy.
Constraints in Experimental Design#
Experiments are powerful but imperfect. Key constraints include:
Ethical considerations: especially in health or policy settings
Cost: running experiments may require incentives or infrastructure
Interference: treatments can spill over between users
Limited sample sizes
Time to measure outcomes
In some scenarios (e.g., earthquakes, economic crises, pandemics), controlled experiments are impossible; forcing analysts to rely on natural experiments or causal inference techniques.