This is an interesting variable, because one could make an argument for it representing a higher level of measurement. For example, if you coded the number of pets, that would clearly be ratio data. If you used a code of 1 for at least one pet and 0 for no pets, that would appear to be at least ordinal. With just a yes/no distinction as presented here, the data are nominal. Psychologists usually prefer to use the highest level of measurement possible. Therefore, they would probably specify the variable here differently--as the number of pets, which produces a ratio scale of measurement.