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The Management Oversight and Risk Tree (MORT)


The Management Oversight and Risk Tree (MORT) is an analytical procedure for determining causes and contributing factors. MORT arose from a project undertaken in the 1970s. The work aimed to provide the U.S. Nuclear industry with a risk management programme competent to achieve high standards of health and safety. Although the MORT chart (the logic diagram that accompanies this text) was just one aspect of the work, it proved to be popular as an evaluation tool and lent its name to the whole programme.

By virtue of public domain documentation, MORT has spawned several variants, many of them translations of the MORT User's Manual into other languages. The durability of MORT is a testament to its construction; it is a highly logical expression of the functions required for an organisation to manage risks effectively. These functions have been described generically – the emphasis is on "what" rather than "how" and this allows MORT to be applied to different industries. The longevity of MORT may also be a reflection of the far-sighted philosophy from which it emerged, a philosophy which held that the most effective way of managing safety is to make it an integral part of business management and operational control.

The MORT programme for assuring safety was written up by W.G. Johnson under the title "MORT: the Management Oversight & Risk Tree" (SAN 821-2, February 19732). Part of this was a method for investigating incidents and accidents that relied upon a logic tree diagram (the eponymous tree of the MORT acronym). The MORT diagram served as a graphical index to Johnson's text, allowing people to apply its contents in a methodical way. To help investigators, especially novices, the original text (which is in excess of 500 pages) was distilled into a forty-two-page question set: the MORT Users Manual3. MORT as a method is now largely independent of MORT as a programme, certainly in Europe. In practice, the MORT text (i.e. SAN 821-2) has become disassociated from the MORT chart, leaving the MORT User's Manual as the most common source of reference.

General Approach

In MORT, accidents are defined as unplanned events that produce harm or damage, that is, losses. Losses occur when a harmful agent comes into contact with a person or asset. This contact can occur either because of a failure of prevention or, as an unfortunate but acceptable outcome of a risk that has been properly assessed and acted-on (a so-called "assumed risk"). MORT analysis always evaluates the "failure" route before considering the "assumed risk" hypothesis.

In MORT analysis, most of the effort is directed at identifying problems in the control of a work/process and deficiencies in the protective barriers associated with it. These problems are then analysed for their origins in planning, design, policy, etc.

To use MORT, you must first identify key episodes in the sequence of events. Each episode can be characterised as:

MORT analysis can be applied to any one or more of the episodes identified; it is a choice for you to make in the light of the circumstances particular to your investigation. To identify these key episodes, you will need to undertake a barrier analysis (or "Energy Trace and Barrier Analysis" to give it its full title). Barrier analysis allows MORT analysis to be focussed; it is very difficult to use MORT, even in a superficial way, without it.

The MORT process is rather like a dialogue between the generic questions of MORT and the situation that you are investigating. You, the analyst, act as the interpreter between MORT and the situation. The questions in MORT are asked in a particular sequence, one that is designed to help you clarify the facts surrounding the incident. Even so, not every question posed by MORT will be relevant on all occasions. Getting acquainted with MORT is essentially about becoming familiar with the gist of questions in this manual. The chart itself then acts as a prompt list allowing you to concentrate on the issues revealed through the process. It is important for you to make notes as you go, just as it would be if you were conducting an interview. In practice, MORT analysts make brief notes on the MORT chart - enough to capture the issues that arise and their assessment of them. To make this process easier to review, it is customary to colour-code the chart as you go: In addition, issues presented by MORT that you judge to be irrelevant, should be crossed-out to show that you have considered them. The outcomes of a MORT analysis are: These results are not gained without effort; one sweep through MORT for one episode is likely to take an experienced MORT analyst about one hour. As a general rule, only use MORT when you judge that it will add to your investigation – do not use it just because you can. Furthermore, you need to be familiar with the method and to have performed it at least once on a real investigation, to be in a good position to make this judgement.

MORT Structure

The top event in MORT is labelled “Losses”, beneath which are its two alternative causes: (1) Oversights and Omissions, or (2) Assumed Risks. All contributing factors in the accident sequence are treated as oversights and omissions unless they are transferred to the Assumed Risks branch. Input to the Oversights and Omissions event is through an AND logic gate. This means that problems manifest in the specific control of work activities, necessarily involve issues in the management processes that govern them.

The Specific and Management branches are the two main branches in MORT. Specific control factors are broken down into two classes: those related to the incident or accident itself (SA1) and those related to restoring control following an accident (SA2). These are under an OR gate because either can be a cause of losses.

MORT is accomplished using the MORT diagrams. As indicated above there are several levels of the MORT diagram available. The most comprehensive, with about 10,000 blocks basically fills a book. There is an intermediate diagram with about 1500 blocks, and a basic diagram with about 300. Of course it is possible to tailor a MORT diagram by choosing various branches of the MORT tree and using only those segments. The MORT is essentially a negative tree, so the process begins by placing an undesired loss event at the top of the diagram used. The MORT user then systematically responds to the issues posed by the MORT diagram. All aspects of the diagram are considered and the “less than adequate” blocks are highlighted for risk control action.

Full application of MORT is reserved for the highest risks and most mission critical activities because of the time and expense required. MORT is also basically a professional tool requiring a specially trained loss control professional to assure proper application. The basic MORT diagram can be used to facilitate and check on the overall hazard ID process by those with the interest and motivation to ensure excellence.

MORT Procedures

  1. Choose an episode from your Barrier Analysis and write it on the MORT chart above SA1 “Incident”

  2. Begin at SB1 ("Harmful energy flow…")

    • State the energy flow above SB1
    • Proceed through chart top to bottom, left to right
    • Code RED or GREEN only with evidence and standard of judgement
    • Code BLUE if evidence or required standard is uncertain
    • Maintain your list of further enquiries as you go
    • Note any provisional Assumed Risks into the table

  3. When SB3 ("Controls & Barriers LTA") completed

    • explore M-branch either by: ad hoc exploration of M-branch or in sequence – a2-MB1, a1-MB1, MA1, MA2, MB2

  4. If needed, select another episode from Barrier Analysis

    • Use fresh MORT chart
    • Repeat steps 3 and 4

  5. When all required SA1 analyses are complete

    • Note on the barrier analysis episodes that have been subject to MORT analysis
    • Move to SA2 – Amelioration
    • Move to M-Branch and explore in the light of the SA2 analysis
  6. Review Provisional Assumed Risks

    • Explore any that are LTA using a1-MB1

  7. Review MB2 in the light of the analysis so far

  8. Review the M-branch issues, taking the overview


The management oversight and risk tree (MORT) is the ultimate hazard ID tool. MORT uses a series of MORT charts developed and perfected over several years by the Department of Energy in connection with their nuclear safety programs. Each MORT chart identifies a potential operating or management level hazard that might be present in an operation. The attention to detail characteristic of MORT is illustrated by the fact that the full MORT diagram or tree contains more than 10,000 blocks. Even the simplest MORT chart contains over 300 blocks. Obviously, full application of MORT is a very time-consuming and costly venture. The basic MORT chart with about 300 blocks can be routinely used as a check on the other hazard ID tools. By reviewing the major headings of the MORT chart, an analyst will often be reminded of a type of hazard that was overlooked in the initial analysis. The MORT diagram is also very effective in assuring attention to the underlying management root causes of hazards.

MORT is the ultimate in ORM hazard ID processes. Unfortunately, in a military context only rarely will the time, resources, expertise, and mission critical issue come together to permit full application of the process. Nevertheless, the wise risk manager will become familiar with MORT processes and will frequently use the basic MORT diagram to reinforce mainstream hazard ID tools.

The MORT diagram is essentially an elaborate negative logic diagram. The difference is primarily that the MORT diagram is already fill-out for the user, allowing a person to identify various contributory cause factors for a given undesirable event. Since the MORT is very detailed, as mentioned above, a person can identify basic causes for essentially any type of event.