Some practical results are derived for population inference based on a sample, under the two qualitative conditions of 'ignorability' and exchangeability. These are the 'Histogram Theorem', for predicting the outcome of a non-sampled member of the population, and its application to inference about the population, both without and with groups. There are discussions of parametric versus non-parametric models, and different approaches to marginalisation. An Appendix gives a self-contained proof of the Representation Theorem for finite exchangeable sequences.