A data processing apparatus and method are provided for generating prediction data. The data processing apparatus has processing circuitry for performing processing operations including high priority operations and low priority operations, events occurring during performance of those processing operations. Prediction circuitry is responsive to a received event to generate prediction data used by the processing circuitry. The prediction circuitry includes a history storage having a plurality of counter entries for storing count values, and index circuitry for identifying, dependent on the received event, at least one counter entry and for causing the history storage to output the count value stored in that at least one counter entry, with the prediction data being derived from the output count value. Further update control circuitry is provided for modifying at least one count value stored in the history storage in response to update data generated by the processing circuitry. The update control circuitry has a priority dependent modification mechanism such that the modification to the at least one count value is dependent on the priority of the processing operation with which that update data is associated. As a result, the prediction data output for a received event associated with a high priority operation is more accurate than the prediction data output for a received event associated with a low priority operation.