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Data recorded during the multipoint line tests which were jointly conducted by American Telephone and Telegraph and International Business Machines Corporation were studied from the point of view of attempting to determine what typical error characteristics are likely to be present during normal operation in a data communication system operating over leased line facilities. The conclusions of this study and the method by which they were reached is given. Error rates were found to be quite time variant. Statistical correlation between errors appeared to be caused by two separate mechanisms. One mechanism is a steady-state electrical noise which accounts for correlation between errors separated by times up to about 5 milliseconds. The second mechanism is a time-varying interference which accounts for correlation between errors separated by as much as several hours.