And so on, for everything you can think of that might mess up your measurement of the effect of one thing on well-being, rather than another.

BBC: Why it's hard to measure happiness

Measurement at the level of total sales dilutes the measured ad effect by including all the repeat purchases.

FORBES: The Downside Of Marketing Mix Models Is There's No Upside For CMOs

Much of the placebo effect seen in clinical trials is the combination of the natural tendency of patients to get better and measurement error.

FORBES: The Medical Power of Ritual