There are two categories of hazard identification techniques: fundamental and comparative methods. Fundamental methods are based on a systematic consideration of deviations from the design intent. The most powerful method is a form of hazard study, which uses Guidewords applied to process stages or functions. Failure Mode and Effects Analysis (FMEA) is the other widely used fundamental method. A primary objective of these techniques is to identify the Initiating Events, which may lead to dangerous situations. Hazard studies may be carried out at various stages as a design evolves. Early in a project, a limited study may be carried out to identify the most serious hazards, which may require consideration of fundamental design changes. Such a study will often be accompanied by some preliminary risk assessment. Later studies will be more detailed, with the objective of discovering all significant hazardous situations. They may also identify operability problems, which may lead to lost production. This involves application of guidewords on a line-by-line basis to plant diagrams and procedures and is known as a Hazard and Operability (HAZOP) Study. Hazard and Operability Studies can be applied to existing plants  in particular when modifications are being considered  but are most effective when carried out at a design stage where a wide range of possible actions still exist. The guidewords used must be relevant to the stage of the design and must be sufficiently comprehensive to be capable of identifying the hazards involved. A general set of guidewords with a broad range of application has been published by I.Chem.E. Many specialist lists have also been developed for particular applications. Experience has shown that this technique is most effective when carried out by a team of designers, operators, safety advisors independent of the design functions and other specialists as appropriate, at a series of study meetings. The outcome of a study meeting is a list of actions to be pursued outside the meetings, e.g. design changes for consideration, cases identified for more detailed study and analytical quantification and items which will require further consideration at a later state in the design. Failure Mode and Effects Analysis (FMEA) involves consideration of the possible outcomes from all known failure modes or deviations within a system, identifying which lead to undesirable situations. There is no formal method but results are usually summarised in tables. FMEA is most useful where there are a limited number of failure modes known to be of interest. When, within this process, the chance of failures and the seriousness of their consequences are ranked to identify the most critical features, the process is known as Failure Modes Effects and Criticality Analysis (FMECA). Hazard identification procedures also provide information on the mechanisms by which the identified hazards can be produced and as such, the distinction between these techniques and the complementary analytical techniques described Section 6.3, which can also be used for hazard identification, is somewhat artificial. The main difference is that the hazard identification techniques do not provide a framework for setting down mechanisms, while the analytical techniques must start from an event, which has been identified by some method. Comparative methods use Checklists based on in-house or industry wide experience and may derive from Codes of Practice or fundamental studies on similar plants. This may be adequate where the plant design is relatively standard and sufficient experience exists for the principal hazards to be well known. Hazard Indices provide identification via checklists, although they also provide a preliminary ranking order for the degree of hazard. The best known and most widely used are the Dow Fire and Explosion Index and the Mond Index.