Most of the figures for Table A1-1 are from the 1976 World Population Data Sheet of the Population Reference Bureau, Inc., Washington D.C. The exception is per-capita energy consumption figures, which are from the Environmental Fund's Population Estimates, 1974 (see below).
Population estimates for many countries undoubtedly contain a large margin of error. In many cases census data are extremely unreliable; in others undetected changes in birth or death rates since the latest census may introduce considerable error into extrapolations. And, of course, in some instances the figures may represent faulty extrapolations from incorrect census data.
The approximate character of available world demographic data was underlined by the appearance since 1974 of population data sheets produced by the Environmental Fund, Washington, D.C., showing strikingly different figures for many countries (including the United States) from those of the U.N. or the U.S. Bureau of the Census. Their explanations for the discrepancies are not implausible and may in many cases be correct. For example, the Environmental Fund includes an estimate of illegal immigrants in its U.S. figures, whereas the Census Bureau and most other organizations do not include them.
On the whole, we believe the figures we are presenting here are the best estimates available. Most of them, of course, should be considered to represent rough magnitudes. A difference of a point or two in birth rates between two countries may have no significance whatever. The most accurate census data come from the DCs; those of many LDCs are extremely suspect. The per-capita gross national product figures should be used with special caution; since population figures and total GNP are both only approximate at best, per-capita GNP figures, a ratio of the two, are especially liable to error. The major features of the current world demographic picture are clear in these data, but in using them their limitations should always be kept in mind.
Table A1-2 shows the population projections of demographer Tomas Frejka in detail (see Chapter 5), demonstrating population momentum.