Observations and measurements are the basis for all data analysis and estimations in forest assessments. In this chapter we are concerned with the response design of forest assessments, sometimes also called observation design, where the following questions are covered: a) what are the types of variables typically assessed in National Forest Assessments; b) how are observations and measurements made on the population elements that have been selected from the population by the sampling design and c) what can be done in forest assessments to achieve high quality data.

Observations and measurements in NFAs are typically taken in the field (be it on field plots or by interviews) or in remote sensing imagery. The decision which variables to observe in a particular NFA is basically a simple one (though in practice some times not so easy): those variables need to be included that are required to generate the target information. All variables need to be defined in an unambiguous manner, and also the measurement procedure must be defined in detail. There needs to be a clearly defined measurement protocol (often referred to as field manual) for the successful implementation of the inventory in the field. The choice and organization of appropriate methods, instruments and tools influence the reliability and quality of the measurements. It is important to train the field crews before the field inventory practice and to supervise and cross-check the field work, measurement devices (calibration), and data delivered.