(a) animals infected with wild-type Newcastle Disease Virus (NDV) or vaccinated with a first vaccine comprising an unmodified mesogenic or lentogenic NDV strain of NDV; and

(b) animals not infected with or vaccinated against NDV and animals vaccinated with a second vaccine comprising an infectious copy of an avian paramyxovirus at least partly derived from NDV obtainable by a process comprising: transfecting at least one cell with an avian-paramyxovirus cDNA comprising a nucleic acid sequence corresponding to the 5′-terminal end of the genome of avian paramyxovirus to generate an infectious copy of an avian pararmyxovirus, wherein said infectious copy of the avian-paramyxovirus encodes one or more viral proteins having a modification relative to wild-type or an unmodified mesogenic or lentogenic NDV strain, said method comprising:

providing a population of animals wherein one or more of the animals in said population has been vaccinated with said second vaccine;

taking at least one sample from an animal in said population to be assigned;

analyzing said at least one sample to determine the presence of antibodies directed against an epitope or marker expressed by an animal infected with wild-type NDV or vaccinated with said first vaccine, but not by an animal vaccinated with said second vaccine;

correlating the presence of antibodies directed against an epitope or marker expressed by an animal infected with wild-type NDV or vaccinated with said first vaccine, but not by an animal vaccinated with said second vaccine, with the animal not having been immunized with said second vaccine; and

assigning the animal to a group.