This article begins with a description of the general structure of state R&D tax credits, and then explains how their true value can be measured, and how the average value as well as the availability of R&D tax credits have evolved over time. It concludes with a discussion of the implications of this evolution in terms of tax competition among states.
Link
http://www.frbsf.org/publications/economics/letter/2005/el2005-26.pdf