In Elsome v. Elsome, 257 Neb. 889, 601 N.W.2d 537 (1999) the trial evidence revealed a true joint physical custody arrangement, because the children lived with their mother 4 days out of the week and lived with their father the remaining 3 days out of the week. That case clearly teaches that child support is to be set on a joint or sole physical custody basis depending on what the trial evidence reveals the arrangement to be-despite what a prior decree or order may have found it to be. Moreover, the doctrine that child support is always subject to modification and is never entirely final has been a part of our jurisprudence for years. See, e.g., Erickson v. Erickson, 202 Neb. 345, 275 N.W.2d 287 (1979). And, in modification matters affecting child support, the paramount concern is the best interests of the children. See, e.g., Schulze v. Schulze, 238 Neb. 81, 469 N.W.2d 139 (1991).