Once scored, indicators can be combined in a variety of ways, such as additive (Andrews and Carroll, 2001), weighted (Karlen et al, 1998), or multiplicative indexes (Doran and Parkin, 1996; Larsen and Pierce, 1991). In a comparison of indexing methods, Andrews et al. (2001) found few differences among index outcomes calculated by differing methods. The Soil Management Assessment Framework index at this site is additive but offers users the option to weight indicator scores.
Andrews, S.S. and C.R. Carroll, 2001. Designing a decision tool for sustainable agroecosystem management: Soil quality assessment of a poultry litter management case study. Ecol. Applic. 11 (6): 1573-1585.
Andrews, S.S., D.L. Karlen, and J.P. Mitchell. 2001. A comparison of soil quality indexing methods for vegetable production systems in Northern California. Agriculture, Ecosystems and Environment 1760: 1-21.
Doran, J.W. and T.B. Parkin. 1996. Quantitative indicators of soil quality: a minimum data set. In J.W. Doran and A.J. Jones, eds. Methods for Assessing Soil Quality. SSSA, Inc., Madison, Wisconsin, USA.
Karlen, D.L. K. Kumar, R.S. Kanwar, C.A. Cambardella and T.S. Colvin. 1998. Tillage system effects on 15-year carbon-based and simulated N budgets in a tile-drained Iowa field. Soil Tillage Res. 48: 155–65.
Larson, W.E., and F.J. Pierce. 1991. Conservation and enhancement of soil quality. In: J. Dumanski, E. Pushparajah, M. Latham, and R. Myers, eds. Evaluation for Sustainable Land Management in the Developing World. Vol. 2: Technical Papers. Proc. Int. Workshop., Chiang Rai, Thailand. 15-21 Sept. 1991. Int. Board for Soil Res. and Management, Bangkok, Thailand.