PHAT: PHoto-z Accuracy Testing

Obtaining redshift from photometry (photo-z), which avoids costly spectroscopy, is now widespread in extragalactic astronomy, especially with large statistical samples. Here the authors present PHAT, the PHoto-z Accuracy Testing programme, an international initiative to test and compare different methods of photo-z estimation. The accuracy of the different methods is expressed and ranked by the global photo-z bias, scatter, and outlier rates. The difference in scatters can reach a factor 2 between models, and there are also a large number of outliers that are detected when comparing the results to spectroscopic redshifts. The test data sets are publicly available and can be used to guide future photo-z method development.