What happens if the respondant is knowledgable about more than one car?
It's not relevant.
I want to know about numbers of cars on the road - which means I want each person in my statistical sample to answer once, and if I get 15 answers about Vauxhall Vectras, then that will reflect the distribution of cars on the road.
Anyone with more than one car should answer about the car they drive most often, or if no such car exists, choose randomly.
no subject
It's not relevant.
I want to know about numbers of cars on the road - which means I want each person in my statistical sample to answer once, and if I get 15 answers about Vauxhall Vectras, then that will reflect the distribution of cars on the road.
Anyone with more than one car should answer about the car they drive most often, or if no such car exists, choose randomly.