Used when universes are very large. It consists of applying a stricter and more systematic methodology in the selection of the sample. After a first random choice, the rest are performed according to established regular intervals, from k to k (elevation coefficient, number of samples contained in the same universe). If the universe is 900 units, for a sample of 30 units, the coefficient k = 30 (30x30 = 900)” (Gaitán and Piñuel, 1998: 152).