# Opt Out From Online Behavioral Advertising By Participating Companies (BETA)

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A Primer on Information Theory and Privacy If we ask whether a fact about a person identifies that person, it turns out that the answer isn't simply yes or no. If all I know about a person is their ZIP code, I don't know who they are. If all I know is their date of birth, I don't know who they are. If all I know is their gender, I don't know who they are. There is a mathematical quantity which allows us to measure how close a fact comes to revealing somebody's identity uniquely. Because there are around 7 billion humans on the planet, the identity of a random, unknown person contains just under 33 bits of entropy (two to the power of 33 is 8 billion). ΔS = - log2 Pr(X=x) Where ΔS is the reduction in entropy, measured in bits, and Pr(X=x) is simply the probability that the fact would be true of a random person. Starsign: ΔS = - log2 Pr(STARSIGN=capricorn) = - log2 (1/12) = 3.58 bits of information Birthday: ΔS = - log2 Pr(DOB=2nd of January) = -log2 (1/365) = 8.51 bits of information How much entropy is needed to identify someone?