stock activities are. per type of aniMal: (a) ~he size of the herd; (D) the genetic co~position of the herd; (c) t~e orientation of the 1 -8 - ~e~d towa~ds a major output such as dairy products or meat; and {d) alte~native m~thoas of livestock raisin ~ with their input requirements tfeedstuffs. labour). As far as tree crop activities are conce~ned, the model assumes knowledge of: {a) the existing stock of tree by kind-; (b) t.heir age dist:dbution; (c) the technolo gy associated with their maintenance bV kin~ and age class; (d) their present and future yield; and (e) the technology of plantlnq. For each commodity, a dema!ld function must lle estimate<l where the quantities demanded depend on the per capita income? the price and the population conside~ed. The pa~ameters of the demand functions -the price and inco111e elasticities -a.-e obtained throuqh an econometric anatysis of the data collected from household expenditure surveys, national accounts and price surveys. The level of disaggregation depends on the commodity and ~ay distinyuish the type of population (ru~aa, urban) and the sub-region .. The level of disaggregation of the data. especialty for pro­ duction, is very de~anding as far as the ~ethods of samp1i.ng and esti­ mation are concerned. Because the o~tput of the model can be very sensititve to a sMall variation in a key variable, it is necessary to have~ together with the estimation of the variabl e, full information on the method of collection and the statistical characteristics of the estimation (size of samp'e• variance of the estimator etc.). On that basis, the planner will be able to build several variants of the model according to the ~ost likety changes in the key variables. To close this brief review of the data requirement s of planning 
**Analysis**
The text appears to be discussing the requirements for a planning model focused on agricultural and forestry activities, with an emphasis on livestock and tree crops. The model seems to be concerned with optimizing production and supply chain management, which suggests a strong connection to economic development and agricultural practices.

**Category**
Agriculture, forestry and fishing, Economic development