In order to determine the evidence upon which the Federal Trade Commission decides that an advertisement or other sales representation has the capacity or tendency to deceive, a census was conducted of the 3,337 cases in which such decisions have been made from 1916 through 1973. Findings showed that, of 22 categories of evidence, the three most frequently used categories were from the internal-commission evidence category--87.2% of the decisions used evidence from that category only. Data also show trends toward using evidence from outside sources; consumer testimony and surveys have been relied upon in more than half of the cases in the 1970s. (JM)