This article is intended to promote a discussion about how one reduces a lot of data to a few representative values while recognizing what gets preserved and what might be lost. Specifically, the phrase, "P10 production forecast" is shown to be ambiguous. By acknowledging this ambiguity and asking the intended use of the values, we examine some possible definitions and illustrate them with examples. The same scrutiny is then applied to P10 values for reserves and NPV, which are presumed to be unambiguous. Although raising more issues than it answers our goal is to open or expand the dialog and to suggest some alternate procedures. Managers as well as analysts need to understand the ambiguity presented below so they can ask better questions and make more informed decisions.

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