As a new lattice problem, we introduce -Shortest Independent Vectors Problem (-SIVP for short), where is a positive integer no greater than the rank of a lattice. In the case where , -SIVP means SVP, and in the case where is the rank of a lattice, -SIVP means SIVP. We estimate upper bounds on the failure probabilities of the reductions from the subset sum problems to the -SIVPs in terms of Ehrhart theory. Especially, in the case of , our upper bound is tighter than the previous result given by Coster et al. We give some considerations for dominating terms of our upper bounds of failure probabilities when is general.