A killer has been stalking streetwalkers in this city .
一个杀人犯一直在这个城市里俏悄地跟踪着妓女。
First , those studies are biased : they tend to be carried out by staff at drop-in centers and by the police , who tend to see the most troubled streetwalkers .
首先,那些研究存在偏差:它们很可能是由收容所的工作人员或员警进行调查的,而这些人易于见到那些麻烦最大的街头妓女。