Every cop and sheriff's deputy dreams of the day that law enforcement can stop crimes rather than solving them after the fact. Now many are buying programs from tech companies like Hitachi and IBM which claim that analyzing big data can predict crime before it happens. But garbage in means garbage out, and critics are asking where all that data comes from. Is it information that beat cops and investigators already know? Is it making law enforcement more efficient and more effective—or reinforcing the biases that are leading to the erosion of public trust?