Although arbitrary, these percentiles represent a compromise between identifying a sufficient number of events for statistical analysis and yet selecting events that may be viewed as extreme or causing an impact. The 99th and 99.9th percentiles are based on the entire record of hourly surge events at a station. There are approximately 5000 hourly surge values in a single surge season. Therefore, on average, the 50 largest hourly surges would exceed 99th percentile and the 5 largest hourly surges would exceed 99th percentile. Since the entire historical record is used not all seasons experience a 99.9th percentile surge.