The following article presents in formal-mathematical language the application of the multi-stage sampling design with its respective estimator in order to build the notion of sampling strategy and use it in a study with a survey to investigate some financial characteristics of micro and small companies (MSC) from the country. The linear unbiased estimator of the sampling techniques used is adapted to the geographical structure of the universe from which the finite population surveyed comes from. Additionally, the notion of sampling strategy is contrasted in light of notable developments in survey sampling theory and theoretical criticisms of the design-based inference approach.