  We show why and when entanglement is needed for quantum-enhanced precision
measurements, and which type of entanglement is useful. We give a simple,
intuitive construction that shows how entanglement transforms parallel
estimation strategies into sequential ones of same precision. We employ this
argument to generalize conventional quantum metrology, to identify a class of
noise whose effects can be easily managed, and to treat the case of
indistinguishable probes (such as interferometry with light).
