One such harm, for example, which I call aggregation, emerges from the fusion of small bits of seemingly innocuous data. When combined, the information becomes much more telling. By joining pieces of information we might not take pains to guard, the government can glean information about us that we might indeed wish to conceal. For example, suppose you bought a book about cancer. This purchase isn’t very revealing on its own, for it indicates just an interest in the disease. Suppose you bought a wig. The purchase of a wig, by itself, could be for a number of reasons. But combine those two pieces of information, and now the inference can be made that you have cancer and are undergoing chemotherapy. That might be a fact you wouldn’t mind sharing, but you’d certainly want to have the choice.
[A great read an why we’re about to make a huge mistake in the US. Reductionism fails again on this issue. Looked at as a whole, it is frightening.]