Oil and gas estimates always involve some amounts of uncertainty. To account for this, the USGS presents its assessments of oil and gas potential in the form of a range based on how certain we are that this amount of oil and gas exist.
The USGS uses a statistically based process (or model) to calculate the likely range of its estimates. The range of values extends from a 5% or greater likelihood of occurrence (the F5 value, or largest estimated value) to less than 95% likelihood of occurrence (the F95 value, or smallest estimated value).
The F50 column is the value that occurs when 50% of the results are greater and 50% are lesser than the model results. We also calculate a mean or average value, which is the number commonly reported in the USGS press releases. The mean is close to, but not identical to, the F50 value.
Learn more: USGS Energy Assessments