Harrison and list:Field Experiments 1017 is that students might be self-selected in allows one to remove this recruitment bias some way,so that they are a sample that from the resulting inference excludes certain individuals with cha racteris Some field ex eriments face a more seri tics that are nts of sa selection tha unde dep problem on the of the task.Once th ev ent has be it is not a importanc he el inf n flo overemphas lways possible to sim th of the .Th matte degree can ead to endoge pop lation are not represented,at nous subject attrition from the experiment least under the tentative assumptio that it is uch attrition is actually informat ive about only observables that matter.In this case it subject preferences,since the subject's exit would behoove the researcher to augment from the experiment indicates that the sub the initial convenience sample with a quota ject had made a negative evaluation of it sample,in which the missing strata were sur (Tomas Philipson and Larry Hedges 1998) veved Thus one tends not to see many con The classic problem of sample selection victed mass murderers or brain surge ons in refers to possible recruitment biases.such that the c where to natur of the ther n in our sa rm2naeraton ent.Th fo Another proble, of any exper pe the pos is th recruitme typ treatmen dures his issue is popu whi s ar es the role o ruited has c vers and plau recruitment fees in biasing the sample sibly expects s the experiment to ave some subjects that are obtained Ihe context for element rand then the her experiment is particularly relevant here observed sample will tend to look less risk- since it entails the elicitation of values for a averse than the population. It is easy to private commoditv She finds that there are imagine how this could then affect behavior some significant biases in the strata of the differentially in some treatments lames population recruited as one varies the Heckman and leffrey Smith (1995)discuss recruitment fee from zero dollars to two dol- this issue in the context of social experi- lars,and then up to ten dollars.An im ortant me nts,but the concern applies equally to finding how r is that field and lab experiments. a s del 4.3 Are Students Different? them. ects and th This wortditdOGo addre ssed in In a group subjects e tre ent ichte stein and Pa ment has 60 cent fem les and the ot (1973) and Penny Burns (1985 sample of subjects in another trea ment has lenn Harrison and James Lesley (1996 only 40 percent females,provided one con (HL)approach this question with a simple trols for the difference in gender when pool statistical framework.Indeed,they do not ing the data and examining the key treatment consider the issue in terms of the relevance effect.This is a situation in which gende might influence the response or the effect of oercholegoaemtche 2 the treatment,but controlling for gender