1. A method performed by data processing apparatus, the method comprising: providing tracer data to a content performance estimation model, the tracer data specifying feature values for a reference content item, the reference content item being a content item having a tracer keyword phrase that has been received from less than a threshold quantity of user devices and for which a reference performance measure is pre-defined; receiving, from the performance estimation model, an estimated performance measure that was computed by the performance estimation model based on the tracer data for the reference content item; and computing an accuracy measure for the content performance estimation model, the accuracy measure being computed using the estimated performance measure and the reference performance measure.