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1 Introduction
The theory of constraints (TOC), conceptualized as a philosophy of continuous improvement, has evolved and expanded its methodological base over time. Originally developed by Dr Eliyahu Goldratt, TOC encompassed a systematic approach to organizational problem solving in the form of "5 focusing steps," first providing a means of identifying the constraining factors preventing a company from achieving its goal through to "breaking" the constraints and repeating the process of improvement. The TOC methodology now comprises three main streams that can be considered as operations strategy tools, performance measurement systems, and thinking process (TP) tools ([21] Cox and Spencer, 1998).
In much the same way as the "5 focusing steps" focus on identifying and managing the constraints on improving performance, the TPs also focus on factors that are currently preventing a system from achieving its goals. As explained later in the paper, the original suite of TPs comprises five logic diagrams (four trees and a "cloud"): current reality tree (CRT), evaporating cloud (EC), future reality tree (FRT), prerequisite tree (PRT), transition tree (TT) and a set of logic rules, the Categories of Legitimate Reservation (CLR). The TPs first identify problematic symptoms which provide evidence that the system is not performing as well as desired. These various TP tools are then used to deduce the causes of those symptoms, what needs to be done to correct those causes, and how such corrective actions could be implemented. Thus, it is claimed, TP tools, using the rigour of cause-and-effect thinking following strict logic rules, could be applied to any kind of problem situation if combined with the intuition and knowledge of the persons owning, or intimately involved with the problem ([61] Mabin and Balderstone, 2000). Therefore, decision makers, if they have knowledge of the TPs, could tackle complex problems with efficiency and effectiveness.
Despite an increasing interest in the TP tools amongst academics and practitioners, there has been no review of the TOC literature that relates to work published since 2000. Even though [86] Rahman (1998) and [61], [62] Mabin and Balderstone (2000, 2003) have already provided reviews of the broader TOC literature, their work was limited to the papers published before 2000. In addition, the recent development of variant TP tools (such as the "three-cloud" approach...