What is a “control” in an experiment?

Because sometimes variables that we are not anticipating to have an effect in our study, it is important to design experiments carefully and to implement various controls.

Here is a famous study by Solomon Asch. We will study this in more detail in Grade 12. If you were designing this experiment, would you use a single-blind or a double-blind design? What would be the advantages and/or disadvantages of both?
What specific confounding variable would be reduced if you used a double-blind study?

Why are placebos getting more effective?