be determined so that diseased individuals are included below the threshold or above the threshold at a first predetermined ratio and healthy individuals are included above the threshold or below the threshold at a second predetermined ratio in an exemplary method a statistical software such as jmp from sas inc can be used to obtain a threshold value with which chi square test yields the best result a diagnostic marker is preferably provided with high values of both the first predetermined ratio and the second predetermined ratio for example the value may be no less than 70 more preferably no less than 80 even more preferably no less than 90 and most preferably 100 when both values are set high specificity as well as sensitivity can be high if it is impossible to set both of the first and second predetermined ratios high the threshold may be determined so that either one of the specificity and the sensitivity becomes high the values of both the specificity and sensitivity are high for example the value may be no less than 70 more preferably no less than 80 even more preferably no less than 90 and most preferably 100 as used herein the specificity refers to a percentage value of the number of healthy individuals above or below the threshold the total number of healthy individuals and the sensitivity refers to a percentage value of the number of diseased individuals below or above the threshold the total number of diseased individuals biomarkers having a high specificity can be suitably used as the biomarker for identifying a patient with depression hereinafter referred to as a disease marker because the probability of a patient being afflicted with depression is high.