Empty zones along a continuum signal causally interesting constraints worth investigating

Evidence

For example, there are good casual reasons why it is hard to achieve absolute zero temperature. The existence of few or zero cases anywhere along the continuum from the negative pole to the positive—for example, in the gray zone between democracy and authoritarianism—generally poses questions worth examination. — Gary Goertz, Social Science Concepts, 578

Categorization

• DEFINE: Clarifies a research heuristic: absences and gaps in the distribution are themselves data demanding explanation.