If you are refering to which type of sperm pathology is most observed in a sperm analysis, you must first indicate which species you refers to, since there are substantive differences in the results between animals that are kept in controlled environments (for example males in breeding centers) with respect to men, even according to the environment in which that men develop. However, in all cases, tail defects are the most common, since the possibilities in which these defects appear is multicausality. Nontheless, since the defects of the sperm head, even appearing in smaller quantities, are more influential in the fertility of that individual.