  Superposition principle of wave function in multi-slit interference
experiment has been widely accepted by many quantum mechanics textbooks,
however the expression ${\psi _{AB}} = {\psi _A} + {\psi _B}$ does not strictly
hold. The non-classical paths in Feynman path integral provide an explanation
to the multi-path interference effect, and can be used to quantitatively
analyse the multi-slit interference effect. In this work, we apply quantities
in Bohmian mechanics to the measurement of multi-order interference by
analysing the contribution of non-classical paths to Bohmian velocity in
electron's multi-slit interferometer. The result shows that the non-classical
paths effect will cause a relative deviation of $10^{-3}$ in electron's Bohmian
velocity, which is observable by doing weak measurement similar to previous
work.
