Measuring the efficiency of chemical reactions is necessary to compare alternative synthetic routes to products in terms of environmental and economic costs.  Percentage yield has historically served this function, as it compares the (predicted) theoretical and actual product quantity.  However, a high yield is not sufficient to identify environmentally-preferable synthetic routes because there can be significant waste generated in a reaction that produces close to the predicted amount of product.  As such, we need an additional metric, atom economy, to measure how much of the reactant atoms actually form the final product whereby:

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This is effectively guiding a chemist to pursue pollution prevention at the molecular scale.  The higher the atom economy, the lower the amount of waste product formed.  Both the yield and the atom economy should be taken into account when designing a green chemical process.