This article considers various kinds of probability and non-probability samples in both experimental and survey studies. Throughout, how a sample is chosen is stressed. Size alone is not the determining consideration in sample selection. Good samples do not occur by accident; they are the result of a careful design. (Author/JAZ)

Student's empirical sampling approach is used to assess the magnitude of the sampling errors of statistics describing a recursive causal model. The data were gathered with four complex sample designs commonly used in educational surveys. Jackknife and half-sample error estimates are applied to the data. (Author/CTM)

It is shown that using formulae for the estimation of sampling errors based on simple random sampling, when a design actually involves cluster sampling, can lead to serious underestimation of error. Jackknife and balanced repeated replication are recommended as techniques for dealing with this problem. (Author/CTM)