Final answer:
To assess the reliability of Punxsutawney Phil's predictions, a record of past weather patterns is the most useful source, as it provides concrete evidence for comparison.
Step-by-step explanation:
To determine the reliability of Punxsutawney Phil's predictions over a number of years, the most useful source would likely be a record of weather patterns in the last decade. This meteorological data would provide empirical evidence of the weather conditions following Phil's predictions and allow one to compare historical trends accurately. A memoir from a member of the Groundhog Club might contain personal insights and historical context, an encyclopedia entry on mammals in the northeast U.S. might provide information about groundhogs, and a historical survey of customs in Pennsylvania might give cultural background, but actual weather data would provide the concrete evidence needed to assess Phil's accuracy.