VIOLATING IGNORABILITY OF TREATMENT BY CONTROLLING FOR TOO MANY FACTORS
研究了在估计处理效应时，若协变量中包含某些因素，会导致关键的处理可忽略性假设被违反，具体考虑了J+1个处理水平且处理随机化但协变量分布因处理水平而异的情况。
This problem shows how the key ignorability-of-treatment assumption used in estimating treatment effects can be violated when certain factors are included among the covariates. The case considered is when there are J + 1 treatment levels, treatment is randomized with respect to potential outcomes, but the distribution of included covariates differs across treatment levels.