First, the study assumed that, in principle, two distinct forms of displacement ought to be detectable as a result of modifying the machines to prevent the use of the 50p slugs: geographic displacement of the 50p slugs from stations with modified machines to those stations where modification had not yet occurred and a shift from 50p to 1- pound slugs following the completion of the modifications. Second, it was assumed that the slugs were used by distinct groups of offenders who would differ in the strength of their motivation and therefore in the degree to which their fraudulent activities might either be stopped or displaced. Subsequent analysis of the distribution of slug use suggested that, although ordinary passengers and delinquents might have been responsible for most of the problem, several groups of organized offenders had been systematically obtaining large sums of money from the machines at distinct clusters of stations. These offenders, therefore, seemed to be the likely culprits when 1-pound slugs, (which are more difficult to make), began to appear soon after the 50p slugs were stopped. Because the same clusters of stations were not affected by the 1-pound slugs, however, the study concluded that these slugs would have appeared anyway and were not simply the result of displacement. Although the remedial measures introduced by the Underground were effective, the problem of slug use could have been anticipated at the design stage and might have been prevented by a greater initial investment in the machines. 7 tables and a map of the Underground