In physics and other sciences, scientists use very pure substances with very precise instruments to reduce the variability in a study. In HCI we can sometimes emulate this, controlling conditions to reduce the variability in our study (reduce noise). For example, in a user test we might ask people to use a mobile phone to perform the same tasks in the same location with similar levels of sound and distractions. The downside of this is that it can sometimes reduce ecological validity, making the resulting conditions less like the real world.
