If an unselected sample includes individuals whose blood pressure is sensitive to their salt intake and individuals whose blood pressure is not sensitive, then the superposition of these two sub-populations in a scatterplot of individuals' blood pressures against their salt intakes could give a triangular distribution. The non-correlation in the insensitives would obscure the correlation expected in the sensitives. This hypothesis justifies truncation of such data to test for correlation between blood pressure and salt intake among only the individuals in the higher range of blood pressures observed. No criterion of salt sensitivity is needed. The analysis should succeed if salt intake makes a major contribution to hypertension and would be improved if other putative causes were factored out.