Sampling intensity
The sample size in this methodology is the number of sample plots where population characteristics are measured. The precision of estimates is in general closely correlated to the sample size, which as it increases, the precision of estimates increases. For equal sample size, the precision of estimates may be improved when stratification on the basis of certain variables of interest is applied. The proposed stratification on the ecological zoning is not likely to improve precision of estimates significantly but rather to apply different sampling intensity to each stratum which leads to different levels of precision among strata.
The sampling intensity proposed to survey the forest and tree resources at the national level is variable from country to country. It should be based within limits of a sample size of 50 to 500 sample sites per country. Each tract contains a cluster of 4 permanent plots of half hectare each. This is deemed to yield, at the national level, estimates of forest and land use attributes at acceptable precision. Precision of less abundant attributes decreases with the decrease of their frequency of occurrence in the country.