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When an investlgator records an observation by nature according to a certain stochastic model the recorded observation will not have the original distribution unless everJ observation is given an equal chance of being recorded. A number of papers have appeared during the last ten years implicitly using the concepts of weighted and size-biased sampling distributions. In this paper we examine some general models leading to weighted distributiosls with weight functions not necessarily bounded by unity. The examples include. probability sampling in sasnple surveys additive damage models visibility bias dependent on the nature of data collection and two-stage sampling. Several important distributions and their size-biased fornls are recorded. A few theorems are given on the inequalities between the mean values of two weighted distributions. The results are applied to the analysis of data relating to human populations and wildlife management. For human populations the following is raised and discussed. Let us ascertain from each snale student in a class the nusHber of brothers including himself and sisters he has and denote by k the number of students and by B and S the total numbers of brothers and sisters. What would be the approximate values of B/(B + S) the ratio of brothers to the total number of children asld (B + S)/k the average number of children per family? It is shown that B/(B + S) will be an overestimate of the proportion of boys among the children per family in the general population which is about halB and similarly (B + S)/k is biased upwards as an estimate of the average number of children per family in the general population. Some suggestiosls are oJ%ered for the estimation of these population parameters. Lastly for the purpose of estimating wildlife population density certain results are formulated within the framework of quadrat sasnpling involving visibility bias. fw(x) = w(x) f(x) (1.1)

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