The polariton-polariton interaction strength is an important parameter for all kinds of applications using the nonlinear properties of polaritons, such as optical switching and single-photon blockade devices. Over the past few years, as experiments with polariton condensates in microcavities have become more sophisticated, there have been several different types of experiments aimed at establishing the absolute value of this parameter. In this paper, we review and compare the results of many of these experiments, and present new theoretical analysis of some of them. The results of measurements sensitive to the polariton scattering rate, though frequently neglected, are especially significant for the low-density estimates. We find that even when adjustments are made to correct for new understanding of past experiments, the values range over two orders of magnitude, with the low values mostly coming from experiments with high polariton density, and the high values coming from experiments at low density. We show that calculation of many-body effects on the effective interaction can give a significant reduction of the effective interaction strength at high density.
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