Countries use different data and methods to calculate imputed rent, as discussed in detail below in the section on “Homeownership in retirement incomes: The concept of imputed rent”. Housing wealth is also less liquid than other assets because owners who wish to liquidate their homes may face transactions costs, frequent problems of indivisibility, and mortgage prepayment penalties. Altogether, housing is more difficult to measure than other types of investments when it comes to turning it into a stream of income. Many studies use actual cost of production to proxy public services’ (monetary) value, as it is relatively easy to determine the cost of the inputs.