Staying with Sarah's site for another post, as she has posted her thoughts on the problem of data quality in football.  All this is motivated by the controversial goal scored by David Villa of Barcelona, and the different offside pictures of the same event published by four Spanish media outlets.  Her post also dovetails with the last two big-picture posts that I've made on soccer analytics and the data collection companies that provide much in-match (and proprietary) data.  The companies have their own procedures and algorithms for collecting data, and some either compile these data through video analysis or human operators watching live matches.  The end-users (i.e. the clubs) should be concerned that their data are dependent on collection processes that are either opaque or highly prone to systematic error, but they aren't at this time.