The 1980 FRA was essentially an expert assessment based on existing inventory information for each of 76 tropical countries. A small team of experts in FAO headquarters gathered and synthesised whatever information they could find regarding the status of forests in each of 76 tropical countries. The diversity of the information was seen in three different angles: subject, scale and reliability. The FRA 1980 study “consisted mainly in the selection, organisation, compilation and interpretation of this abundant and diverse mass of information using a single framework of classifications and concepts for the 76 tropical countries” (Lanly 1982). Although it was clearly the best available global synthesis of its time, the 1980 FRA was criticised for over reliance on country-reported data, which was feared to be biased in favour of underreporting rates of forest loss. Another criticism was that the methods used to synthesise and update data were not fully documented and made available to potential users of the data.
Partly due to this, as well as to the increased staff capacity and access to technology, the 1990 FRA incorporated a statistical sample of satellite imagery to produce regional levels of forest cover change, as a validation of the aggregated country level results produced by country data and models. This survey covered, with 10 percent intensity, all tropical forests in wet, moist and dry conditions. The survey was based on a statistical sample composed by multi-temporal satellite images over 117 sampling units. For each unit the study included the analysis of one entire Landsat image close to year 1990 and one (or more) close to year 1980.
The 1990 FRA also used a set of models to predict forest change as a function of population, based on the assumption that “the growth of human population, independently in the absence of economic development, and jointly with poorly planned and uncontrolled economic development, are the driving forces behind human activities that initiate deforestation” (Scotti 1990, Palo 1986). It was assumed that forest area reduction increases slowly at initial stages, faster in intermediate stages and slows down at final stages, following principles of biological growth processes, often described with well-known Chapman-Richards function. The mathematical connection between population density and non-forest area was created using forest cover data in a number of sub-country regions in Tropical countries. Finally, the deforestation rate was estimated assuming that the increase in the population will determine the decrease in the change of forest cover.