The positivity constraints for hadronic tensor lead to the bounds for \tau lepton polarization in Parity Violating DIS. Another model-independent constraints are provided by the QCD duality which may be described in a similar way to QCD sum rules method. The Parity Conserving spin-dependent case is considered in more detail, while several specific comments on Parity Violating case are made. The observation is made that {\Delta }(1232) resonance should be excluded from duality consideration and possible reason for that is offered.

PACS numbers: 12.15.Ji, 13.15.+g, 14.20.Gk, 23.40.Bw