Categorisation of action slips in human error Norman, 1981

This post is part of a series of notes I collated during my studies at UCL’s Interaction Centre (UCLIC).

A slip is an error that occurs when a person does an action that is not intended.

The path from intention to action consists of the activation of the parent schema that corresponds to the intention, the activation of the child schemas for the component parts of the action sequence and then the appropriate triggering of schemas when the conditions match those required for their operations.

The three major categories of slips are:

  • Errors in the formation of the intention, which includes the subcategories of mode and description errors;
  • Faulty activation of schemas, which includes the subcategories of capture errors, data-driven and associative actions, loss of intention and misordering of action components; and
  • Faulty triggering, which includes the subcategories of spoonerisms, blends, intrusions of thoughts and premature triggering.

One interesting aspect of slips is peoples’ ability (or inability) to detect them. Many slips are caught at the time they are made. Sometimes they are caught just prior to their occurrence, but with insufficient time to prevent the act, or at least the initial stages of the act.

For a slip to be started, yet caught, means that there must exist some monitoring that is separate from that responsible for the selection and execution of the act.

A schema is an organised body of knowledge, including procedural knowledge that can direct the flow control of motor activity.

Each schema is assumed to cover only a limited range of knowledge or actions. As a result, any given action sequence must be specified by a rather large ensemble of schemas, organised in a hierarchical control structure.

One schema may need to invoke other schemas, passing to them particular values that the variables of the schemas must assume for the particular actions to be performed.

Information passes both down from the higher-order schemas to the lower ones and back up from the lower-order schemas to higher ones.

Numerous schemas will be activated at any given time. Implications:

  • Any given action sequence is usually quite complex, involving a large number of component schemas.
  • Many (most) action sequences require considerable time to be completed. Multiple intentions and schemas are usually active at any given time. The determination of the appropriate triggering conditions for a given schema is a critical factor in the correct performance of the act.

Schemas only invoke actions when they have been triggered and this requires satisfaction of trigger conditions plus a sufficiently high level of activation.

The theory of action permits numerous opportunities for slips:

  • Errors in the selection of the intention.
  • Errors in the specification of components.
  • Errors of performance when schemas are triggered out of order or when a relevant schema is missed.
  • Errors resulting from the intrusion of unwanted activities and thoughts.
  • Errors resulting from the occurrence of an event that triggers an unintended response.
  • Errors resulting from a well learned, familiar habit taking control of the action.
  1. Slips that result from errors in the formation of the intention.

    • Errors that are not classified as slips; errors in the determination of goals, in decision making an problem solving and other related aspects of the determination of an intention.
    • Mode errors: erroneous classification of the situation.
    • Description errors: ambiguous or incomplete specification of the intention.
  2. Slips that result from faulty activation of schemas.

    • Unintentional activation: when schemas not part of a current action sequence become activated for extraneous reasons, then become triggered and lead to slips.
      • Capture errors: when a sequence being performed is similar to another more frequent or better-learned sequence, the latter may capture control.
      • Data-driven activation: external events cause activation of schemas.
      • Associative activation: currently active schemas activate others with which they are associated.
    • Loss of activation: when schemas that have been activated lose activation, thereby losing effectiveness to control behaviour.
      • Forgetting an intention (but continuing with the action sequence).
      • Mis-ordering the components of an action sequence.
      • Skipping steps in an action sequence.
      • Repeating steps in an action sequence.
  3. Slips that result from faulty triggering of active schemas.

    • False triggering: a properly activated schema is triggered at an inappropriate time.
      • Spoonerisms: reversal of event components.
      • Blends: combinations of components from two competing schemas.
      • Thoughts leading to actions: triggering of schemas meant only to be thought, not to govern action.
      • Premature triggering.
    • Failure to trigger: when an active schema never gets invoked
      • The action was pre-empted by competing schemas.
      • There was insufficient activation, either as a result of forgetting or because the initial level was too low.
      • There was a failure of the trigger condition to match, either because the triggering conditions were badly specified or the match between occurring conditions and the required conditions was never sufficiently close.

For a slip to be detected, the monitoring mechanism must be made aware of the discrepancy between intention and act.

The basic control sequence is from intention to triggering the action. The only way for an error to be detected is for it to occur within the action triggering mechanism or in the actual mechanics of performing the response.

Each level of the specification of the intention must be decomposed into more basic levels in order for an action to take place, each new decomposition more finely dividing the actions required and more precisely specifying what must be done. And each new level of specification is, in turn, decomposed into its basic components, until some primitive level of act specification is reached. Feedback and monitoring is required at each level.

Updated on: 10 February 2021

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