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100 Chapbr 3 Understanding users

Cognitive tracing invdves externally manipulating items into different orders
or structures.

Annotating is often used when people go shopping. People usually begin their
shopping by planning what they are going to buy. This often involves looking in
their cupboards and fridge to see what needs stocking up. However, many people
are aware that they won't remember all this in their heads and so often externalize
it as a written shopping list. The act of writing may also remind them of other items
that they need to buy that they may not have noticed when looking through the
cupboards. When they actually go shopping at the store, they may cross off items
on the shopping list as they are placed in the shopping basket or cart. This provides
them with an annotated externalization, allowing them to see at a glance what
items are still left on the list that need to be bought.
Cognitive tracing is useful in situations where the current state of play is in a
state of flux and the person is trying to optimize their current position. This typi-
cally happens when playing games, such as:
in a card game, the continued rearrangement of a hand of cards into suits, as-
cending order, or same numbers to help determine what cards to keep and
which to play, as the game progresses and tactics change
in Scrabble, where shuffling around letters in the tray helps a person work
out the best word given the set of letters (Maglio et al., 1999)

It is also a useful strategy for letting users know what they have studied in an online
learning package. An interactive diagram can be used to highlight all the nodes vis-
ited, exercises completed, and units still to study.
A genera1 cognitive principle for interaction design based on the external cog-
nition approach is to provide external representations at the interface that reduce
memory load and facilitate computational offloading. Different kinds of informa-
tion visualizations can be developed that reduce the amount of effort required to
make inferences about a given topic (e.g., financial forecasting, identifying pro-