You have provided an excellent overview of how the household registration system works. I already knew most of that, which was the basis of my question. Your explanation bolsters my question, rather than answers it. We are talking about how they calculate the ratio of "cars per household" in a prefecture. To calculate that, they need only two figures: a) the number of cars registered in the prefecture (of the type being considered), and b) the number of households in the prefecture. You say that the koseki information is not accessible to the public. That's not needed - presumably the number of koseki per prefecture is published somewhere, and that's all that would be needed, if that were to be the basis of the calculation. Also, you say that auto registration address is used to determine household. That would be an excellent source of information to a) count the number of cars registered in each prefecture, and b) how many households in the prefecture have at least 1 car registered to it, but how are you going to count the number of households that have no cars registered to them by using the auto registration data? You have to have this last number in order to calculate the number of cars per household. And we know that they are including households with no cars in that calculation, otherwise the ratio would never be less than 1.0.