Defining high reliability and specifying what constitutes a high reliability organization has presented some challenges. Roberts initially proposed that high reliability organizations are a subset of hazardous organizations that have enjoyed a record of high safety over long periods of time. Specifically she argued that: “One can identify this subset by answering the question, “how many times could this organization have failed resulting in catastrophic consequences that it did not?” If the answer is on the order of tens of thousands of times the organization is “high reliability”” (p. 160). More recent definitions have built on this starting point but emphasized the dynamic nature of producing reliability (i.e., constantly seeking to improve reliability and intervening both to prevent errors and failures and to cope and recover quickly should errors become manifest). Some researchers view HROs as reliability-seeking rather than reliability-achieving. Reliability-seeking organizations are not distinguished by their absolute errors or accident rate, but rather by their “effective management of innately risky technologies through organizational control of both hazard and probability” (p. 14). Consequently, the phrase "high reliability" has come to mean that high risk and high effectiveness can co-exist, for organizations that must perform well under very trying conditions, and that it takes intensive effort to do so.