Harrison and List:Field Experiments 1015 average treatment effect is given by individuals with the same value for these fac- y*o where y*and y*are the treat- tors will display homogenous responses to ed and nontreated Iaverage outcomes afte the treatment,then the treatment effect can the treatment.We have much more to be measured without bias.In effect,one about controlled exp eriments,in particulan use statistical methods to identify which field experime ts.bel individuals are s lab rat Na experiments"the treat- for th eatme ent and find a na fo rmally.th tha th compar Z)∈(0，l,where terenc outcome lenotes independence after rfor the treate group with the before Another alterative to the DID model is and after outc nes for the nontreated group. the use of instrumental variables(IV),which Estimation of the treatment effect takes the approaches the structural econometric form Y=XB+tT.+n.where i indexes the method in the sense that it relies on exclusion unit of observation.t indexes vears.y.is the restrictions (Joshua D.Angrist,Guido W outcome in cross-section i at time t,X is a Imbens and Donald B Bubin 1996.and vector of controls,T.is a binary variable Joshua D.Angrist and Alan B.Krueger 2001) n=a++E.and t is the difference-in-dif The IV method,which essentially assume ferences(DID)ave age treatment effect.If that some components of the no we assume that data exists for ntal data are he then t=(y* utilized for ome 6 g 2000).Th to find variab of theg nd Kenn ed from proa the e outcome equation ut whi -specific sh ch is relate to unit to ent status and has no direct association are correlated with treatment status with the outcome.The weakness of the IV and that selection into treatment is inde approach is that such variables do not often pendent of temporary individual-specific exist.or that unpalatable assumptions must effect:E(maXwD)=E(aIX,D)+入，If be maintained in order for them to be used to E,and t are related,DID is inconsistently identify the treatment effect of interest. estimated as E(t)=t+E(Er-Eo D=1) A final alternative to the DID model is -E(e,-￡oD=0) structural modeling.Such models often entail One alte native method of assessing the a heavy mix of identifying restrictions (e.g. impact of the treatment is the method of ensity and dor 1(1983) ed in eaker ethod has be nsively the debat nd ence nt effec uld have been had the that sele 100g and Te that the di Smith and Petra Todd(2000). en matchi goa dbymay limit the use PSM is to make no perimental data baum and（a9s3, 9s4)sh d that matchn like" experimental data.The intuition to g behind PSM is that if the researcher can treated p(Z). the select observable factors so that any two