What is the Iron Triangle, and how has it changed?

Purpose - The Iron Triangle, also called the Triple Constraint, is a central concept to project management research and practice, representing the relationship between key performance criteria. However, there is disagreement about which criteria should be represented on the vertices of this triangle. The purpose of this paper is to explore which concepts are part of the Iron Triangle, and how these concepts have changed over time. Design/methodology/approach - This paper explores 45 years of project management research, drawing on a database of 109,804 records from 1970 to 2015. Three corpora were constructed, representing the project management and Time, Cost, and Quality Management literature. Time and Cost are consistently identified as part of the Iron Triangle. However, the status of quality is contested. Key concepts in the project management literature were explored using scientometric research techniques, to understand the relationship between these concepts. Findings - Significant links were found between Time, Cost, and Quality, verifying these concepts as the vertices on the Iron Triangle. These links were significantly stronger than links to alternatives, such as Scope, Performance, or Requirements. Other concepts that are core to the Iron Triangle were also identified, and how these have changed over time. Originality/value - This research develops the understanding of a key project management concept by clarifying which concepts are part of the Iron Triangle, based on evidence of how the concept is used in research. This paper also reveals the context in which this concept is used, and how this has changed over the last 45 years.