Special instance of selective sampling where no randomisation is performed in extracting the sample but units are selected only on the basis of feasibility or ease of data collection. Sampling used in exploratory research where the researcher is interested in getting an inexpensive approximation of the truth. The subpopulations may or may not be determined on a risk basis. The sampling from each subpopulation may not be proportional: the sample size is proportionally bigger for instance in subpopulations considered at high risk. This methodology is potentially subject to serious bias.