Toimintaverkoista ja niiden käytöstä maatalouden työteknisissä tutkimuksissa

Authors

• Erkki H. Oksanen Maatalouden työtekniikan laitos, Helsingin Yliopisto

DOI: https://doi.org/10.23986/afsci.71739

Abstract

An activity network is a visual model of a project drawn on a plane where the different activities of a project and their time dependences are presented. Resource and/or cost calculations may also belong to a network. The best known activity networks, CPM and PERT, were developed in the U.S.A. towards the end of the 1950’s. The main difference between the two was that CPM used one duration estimate per activity while PERT used three. CPM concentrated on cost minimization of the project, PERT on the determination and control of the time required by a project development. Later the differences between the two methods have been much reduced. In this article, the planning of networks and their calculation by hand are presented with the aid of a hypothetical example of a research project (Table 1, Figs. 1 and 2). The network reveals the critical path of a project. The path goes through the network along the chain of activities that has the smallest slack (float); in general it is = 0. When attempts are made to shorten the critical path the near critical activities should also be considered since they may easily turn into bottleneck activities. The author has used the PERT C-programme of the State Computer Centre in the study of two building projects and a drainage project, and spring field work methods. In the last mentioned study (Fig. 3) the different working methods, i.e. work with separate machines, work with a row fertilizer + seed dirll, and work with a combined fertilizer drill, have been found about equal at the Viik Experimental Farm as far as completion of the work is concerned. The row fertilizer method was the cheapest while the method using a combined fertilizer drill involved the highest costs. Man and tractor resources have been loaded at their heaviest when using separate machines. The activity network with weather forecasts made in the winter has tallied well with the starting and completing dates of the work, and with the critical path. With the computer outputs from the drainage and building projects (Fig. 4, Tables 2—4), the use and adjusting of the resources have been explained, as well as the calculation of the costs. The suitability of the network models as basis for simulation and gaming is mentioned at the end of the article. They are assumed to become more common when enough basic data is available. The present day activity networks are easily applicable to agricultural technology studies which deal with the rational use of manpower, machinery and equipment on a farm.