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There is some debate about how long items in long-term memory are stored. Some
theories propose that items decay to the point that they are no longer retrievable, and
other theories propose that the items are still there, but no longer retrievable because
they are no longer unique enough to be retrievable. The implications of these two
different approaches may be basically the same for system design.
5.2.1.5 Declarative Versus Procedural Memory
Descriptions of memory often divide memory into two types to illustrate different
effects. Perhaps the most common is the difference between declarative and pro-
cedural memory, which is used to categorize their contents based on how they are
used.
The contents of declarative memory are facts or statements about the world,
such as ''The hippy is in the park,'' ''the star is above the square,'' and ''Colleen's
hair is auburn.'' Retrieval of items from declarative memory is improved by
practice, and the items are intentionally generated for sharing and verbalizing. One
study estimated that it takes about 6 s of processing to encode an item for sub-
sequent retrieval (Simon 1974 ), although the time required is context dependent:
more complex items may take longer.
Declarative memory is used to store and retrieve information such as simple
user instructions, user passwords, and to understand materials in interfaces.
The contents of procedural memory are acts, or sequences of steps that describe
how to do particular tasks. These items can be viewed as a type of programming
language for cognition. Examples of items that would be stored in procedural
memory include how to type, how to ride a bicycle, and many aspects of how to
program or use an interface.
Items in procedural memory are generally more robust against decay, and
retrieval is often less context sensitive than items in declarative memory (Jensen
and Healy 1998 ). Like declarative memory, retrieval and application gets faster
with practice.
The ACT-R theory (a unified theory of cognition, or ''UTC,'' realized as a
computer program, and explained earlier in the introduction and used in the
concluding chapter) uses these two types of memory explicitly, both procedural
(rules) and declarative (chunks). The Soar theory (another UTC) represents
declarative information as the result of a procedure to retrieve information from
memory, and is thus a type of procedural memory.
5.2.1.6 Implicit Versus Explicit Memory
Memories can also be categorized as explicit or implicit: items stored in explicit
memory are reportable, whereas items in implicit memory are not. Most declar-
ative information is explicit in that it can be reported, whereas most procedural
information is implicit in that the precise details are not reportable.
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