The levels of economic activity and growth, despite complex conceptual and technical problems, are reasonably well measured in the economically more advanced nations of the world. In such countries statistics relating to economic activity are generally available, including data on national income, industrial production, agricultural production, employment, size and composition of the labor force, non-human energy utilized, transport and communication facilities, and the like. Where such direct measurements are available, population statistics need not be resorted to as "indicators of economic development". On the contrary, the economic data are often used as independent variables to help explain the significant demographic changes that have occurred as concomitants of economic development.