摘要

  In this paper, we discuss contingency tables with nonresponses.    

  Nonresponse mechanisms can be classified into three types: 

    (1) missing completely at random (MCAR), 

    (2) missing at random (MAR) and 

    (3) nonignorable nonresponse mechanisms. 

  The MCAR and MAR are ignorable models. This paper uses decomposable graphical models to describe the mechanisms of nonignorable nonresponse and discusses identification of parameters in these models. For an unidentifiable model, we propose adding covariates which are always observed such that the model becomes identifiable.