Assigning probability values to errors is still very much an art form. Nonetheless, there is sufficient general agreement about the likelihood of errors occurring in various generic tasks to put them into sensible rank order. Their main use is to provide benchmarks for the range of error probabilities over different jobs.
Here is a list of several well-researched error-enforcing conditions (EEC). For each one, there are the following:
- Description: a general description
- Risk factor: a number (x10) representing the approximate amount by which the nominal error probability will increase when someone encounters this EEC initially (the error probability cannot exceed 1.00)
- Error type: the most likely types where these can be predicted on psychological grounds. The conditions are listed in order of severity, beginning with the most provocative conditions.
| Error-Producing Situations | |||
| Category | Context | Risk Factor |
Type |
| Unfamiliarity | A novel and potentially dangerous situation | 17 | KB Mistake |
| Time shortage | Time available for error detection or correction | 11 | All Types |
| Noisy Signals | Low signal-to-noise ratio in communications between groups | 10 | Misperception |
| Poor interface | Equipment fails to communicate vital signals or conflicting controls | 8 | Action Slips |
| Designer/User mismatch |
Equipment conflicts with the user’s model of the world | 8 | All Types |
| Irreversibility | Unforgiving equipment items can’t correct actions | 8 | All Types |
| Overload | Too much information was badly presented | 6 | Perceptual slips |
| Technique unlearning |
need to apply a new technique having reversed action | 6 | strong habit intrusions |
| Knowledge transfer |
Need to transfer knowledge from task to task without loss | 5 | strong habit intrusions |
| misperception of risk |
Mismatch between real and perceived risk | 4 | Mistakes |
| Inexperience | Training or inexperience insufficient | 3 | Mistakes |
| Checking | Little or no checking of job done | 3 | Undetected errors |
| Culture | macho culture, dangerous procedures | 2 | Accident |
| Morale | morale low, loss of faith in leaders | 1.2 | Sloppiness |
| Monotony | Boredom, repetitious tasks | 1.1 | Unvigilant |
| External Pacing | Pressure of supervisors | 1.06 | Slips |
| ERROR-ENFORCING CONDITIONS AND THEIR RELATIVE EFFECTS | ||
| Generic Task | Nominal Error Probability | |
| 1 | Carrying out a totally novel task with no real idea of the likely consequences. | 0.75 (0.50–1.00) |
| 2 | Attempting to repair a fairly complex system on a single attempt without direct procedures or the procedures to hand. | 0.30 (0.20–0.30) |
| 3 | Performing a complex task requiring a high degree of knowledge and skill. | 0.20 (0.10–0.35) |
| 4 | Performing a moderately simple task quickly and with scant attention. | 0.10 (0.06–0.28) |
| 5 | Restoring a piece of fairly complex equipment to its original state following maintenance. | 0.007 (0.0008–0.010) |
| 6 | A highly familiar routine task performed by a well-motivated and competent workforce. | 0.0005 (0.00008–0.0010) |