Abstract: It is common that geotechnical samples are delimited according to the natural geological origin and period. The necessity of dispersion of geotechnical sampling probability distribution, and the degree of diversity is relative to partitioned range of the base layer are expounded. Based on the analysis result of the applicability of variation coefficient and the generality of sampling, the use of the resemblance index to appraise the degree of the dispersion is advanced, and the number of geotechnical samples must be strictly controued according to the principle of relative capacity. The author suggests, according to the essence of the base layer to infer the risk limit data, should be by another method in a small amount samples condition.