How do you determine the degree of static indeterminacy of a structure?

For plane trusses: Degree of indeterminacy = m + r - 2j, where m = number of members, r = number of reactions, j = number of joints. For frames: DI = 3m + r - 3j for rigid-jointed frames, accounting for internal member forces and moments. A structure is determinate if DI = 0, indeterminate if DI > 0, and unstable if DI < 0 or has geometric instability despite adequate supports.