Staff could study a mass of printouts to find which links and crosspoints (out of, in some offices, a million crosspoints) were causing calls to fail on first tries. In the late 1970s, teleprinter channels were gathered together in Switching Control Centers (SCC), later Switching Control Center System, each serving a dozen or more 1ESS exchanges and using their own computers to analyze these and other kinds of failure reports. They generated a so-called histogram (actually a scatterplot) of parts of the fabric where failures were particularly numerous, usually pointing to a particular bad crosspoint, even if it failed sporadically rather than consistently. Local workers could then busy out the appropriate switch or grid and replace it.