In social networks, not all relationships are created equal. You might cosign a loan application for your sibling, but probably not for your cousin's babysitter's dentist's chimney sweep. In sociology, the strength of a relationship is captured by the concept of tie strength. In this context, a tie is some kind of an interpersonal relationship, and the strength is any measure of how intense or intimate that relationship is.
In 1973, the sociologist Mark Granovetter described the importance of weak ties in bridging different communities. If all ties within a community are strong, then any ties between communities must be weak. He described this phenomenon as the strength of weak ties. By bridging different communities, weak ties make it possible to find information from distant parts of a network. But how do we measure tie strength?
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