Data prefer negative running; this preference remains when additional parameters are freed. We show the marginalized two-dimensional posteriors for n _s (at k = 0.025 Mpc ⁻¹) and dn _s / dln  k. Left panel: the n _s − dn _s / dln  k joint distribution in the ΛCDM+ dn _s / dln  k model for different combinations of the data sets. Right panel: the n _s − dn _s / dln  k joint distribution obtained by marginalizing N _eff (red) and Y _p (blue) for WMAP 7 +SPT data. The purple filled contours show the case with Y _p marginalized with the prior Y _p < 0.294, the 2σ upper limit of the solar initial helium abundance by Serenelli & Basu (2010). We show the region where | dn _s / dln  k| < (1 − n _s ) ² in gray. Inflation models with slow-roll expansion that can be terminated at second order make predictions in this region.