A method for enriching relatively short nucleic acid from a nucleic acid composition, which nucleic acid composition is a cell-free biological composition or a substantially cell-free biological composition, which method comprises: (a) contacting cell-free nucleic acid from a cell-free biological composition or a substantially cell-free biological composition with a solid phase under association conditions, wherein: (i) the cell-free nucleic acid of the cell-free or substantially cell-free biological composition comprises relatively short nucleic acid and relatively long nucleic acid, (ii) the relatively short nucleic acid is 300 base pairs or less, and (iii) the relatively long nucleic acid is larger than 300 base pairs, and (iv) the association conditions comprise a C1-C6 alkyl alcohol and do not comprise a volume exclusion agent; whereby the relatively short nucleic acid and the relatively long nucleic acid are associated with the solid phase; (b) introducing the solid phase after (a) to dissociation conditions that comprise a volume exclusion agent and a salt, wherein: (i) the salt comprises an ionic salt and not a chaotropic salt, and (ii) the relatively short nucleic acid preferentially dissociates from the solid phase under the dissociation conditions as compared to the relatively long nucleic acid, thereby yielding dissociated nucleic acid; and (c) separating the dissociated nucleic acid from the solid phase, whereby the relatively short nucleic acid is enriched in the dissociated nucleic acid relative to in the cell-free or substantially cell-free biological composition, and wherein said substantially cell-free biological composition includes up to 50 cells per milliliter. (a) contacting cell-free nucleic acid from a cell-free biological composition or a substantially cell-free biological composition with a solid phase under association conditions, wherein: (i) the cell-free nucleic acid of the cell-free or substantially cell-free biological composition comprises relatively short nucleic acid and relatively long nucleic acid, (ii) the relatively short nucleic acid is 300 base pairs or less, and (iii) the relatively long nucleic acid is larger than 300 base pairs, and (iv) the association conditions comprise a C1-C6 alkyl alcohol and do not comprise a volume exclusion agent; whereby the relatively short nucleic acid and the relatively long nucleic acid are associated with the solid phase; (i) the cell-free nucleic acid of the cell-free or substantially cell-free biological composition comprises relatively short nucleic acid and relatively long nucleic acid, (ii) the relatively short nucleic acid is 300 base pairs or less, and (iii) the relatively long nucleic acid is larger than 300 base pairs, and (iv) the association conditions comprise a C1-C6 alkyl alcohol and do not comprise a volume exclusion agent; whereby the relatively short nucleic acid and the relatively long nucleic acid are associated with the solid phase; (b) introducing the solid phase after (a) to dissociation conditions that comprise a volume exclusion agent and a salt, wherein: (i) the salt comprises an ionic salt and not a chaotropic salt, and (ii) the relatively short nucleic acid preferentially dissociates from the solid phase under the dissociation conditions as compared to the relatively long nucleic acid, thereby yielding dissociated nucleic acid; and (i) the salt comprises an ionic salt and not a chaotropic salt, and (ii) the relatively short nucleic acid preferentially dissociates from the solid phase under the dissociation conditions as compared to the relatively long nucleic acid, thereby yielding dissociated nucleic acid; and (c) separating the dissociated nucleic acid from the solid phase, whereby the relatively short nucleic acid is enriched in the dissociated nucleic acid relative to in the cell-free or substantially cell-free biological composition, and wherein said substantially cell-free biological composition includes up to 50 cells per milliliter.