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Large blackouts typically involve the cascading outage of transmission lines. We estimate from observed utility data how much transmission line outages propagate, and obtain parameters of a probabilistic branching process model of the cascading. The branching process model is then used to predict the distribution of total number of outages for a given number of initial outages. We study how the total number of lines outaged depends on the propagation as the cascade proceeds. The analysis gives a new way to quantify the effect of cascading failure from standard utility line outage data.