Guide Words

Guide words can be used to identify how activities can fail in a systematic way. Although the methodology (HAZOP) was originally designed for industrial processes, it can also be used to identify possible failure modes of other types of activity.

Method of Use

  1. For a particular activity, identify a series of parameters or attributes related to activity and record them.
  2. For each of the parameters or attributes, ask whether there is a way for the parameter to deviate in accordance with each of the guide words.
  3. Determine the probability and likely consequences of such a deviation occurring.
  4. Document the results.
Common Guide Words
Guide Word Meaning
NO OR NOT Complete negation of the design intent
MORE Quantitative increase
LESS Quantitative decrease
AS WELL AS Qualitative modification/increase
PART OF Qualitative modification/decrease
REVERSE Logical opposite of the design intent
OTHER THAN Complete substitution
EARLY Relative to the clock time
LATE Relative to the clock time
BEFORE Relating to order or sequence
AFTER Relating to order or sequence


As an example, let's consider how a building's Heating, Ventilation and Air Conditioning (HVAC) system might fail:

Parameters (for this example): air temperature, power supply voltage.

Using this method of combining guide words with parameters / attributes forces us to consider the many ways any process or activity can fail.

Further Reading

More information on the HAZOP methodology and the use of guide words can be found on Wikipedia.

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