19. A computer-readable storage device having stored thereon instructions, which, when executed by data processing apparatus, cause the data processing apparatus to perform operations comprising: receiving a plurality of training examples; training a plurality of different types of predictive models using the received training examples, wherein each of the predictive models implements a different machine learning technique; measuring the performance of each trained model; computing a suggestion score for each training example according to each respective trained model, wherein the suggestion score is based on one or more factors that indicate whether additional examples similar to the training example would improve the performance of the model, including weighting each suggestion score by the measured performance of the respective trained model; combining the computed suggestion scores for each training example to compute an overall suggestion score for each training example; and ranking the training examples by overall suggestion scores.
20. The storage device of claim 19 , wherein the operations further comprise: providing one or more of the highest-ranked training examples to a user as a suggestion to add examples similar to one or more highest-ranked training examples.