Previous models of planning like Sacerdoti (1975) tend to be successive-refinement models. This model shares the view that the planning process happens on a 2d space of time and abstraction.
Presented is a protocol of planning. In it the subject changes levels of abstraction throughout. At one point the subject notices that many errands in the task occur in same area, so the subject makes a higher level plan to go to that area and do all of those errands at the same time. The first thing the subject does in place all items in the plan into 2 categories, primary and secondary, according to importance. The subject does not plan strictly forward in time either. The subject sometimes works on the details of subplans. The subject also uses mental simulation to get information about how long things should take, etc. (p500).
This paper presents an opportunistic model of planning. Planning is done by a bunch of specialits (akin to demons in Selfridge's Pandemonium model (1959)). All specialists record decisions on a shared memory structure called a blackboard. Specialists make decisions based on heuristics and what is on the blackboard.
The blackboard is broken up into planes, each with different conceptual information (e.g. explicitly planned activities, decisions about data.) Each plane has multiple levels of abstraction. Most specialists deal with only a few levels of a few planes.
Specialists are like production rules. They have complex preconditions and the postconditions always result in a new or changed decision. The blackboard contains 5 conceptual planes: plan, plan abstractions, knowledge base, executive and meta plan.
plan plane: specific actions the planner will carry out.
plan abstractions plane: desired attributes of potential plan
decisions. (e.g. "go to the nearest errand next")
Abstraction levels: (each cooresponds to the levels of the plan plane.)
knowledge base plane: observations and computations about the
world. (e.g. "the florist is closest to the health club.") (each
cooresponds to the levels of the plan
executive plane: decisions about the allocation of cognitive
resources. (which part of the plan will we develop now? Which
specialists will do it?)
meta plan plane: the planner's understanding of the problem, how to
evaluate the plan, etc.
There were differences, though. Part of the reason for this is that the model makes random choices among specialists when many are of equal value. Also. the list of specialists is incomplete.
People do not work completely top-down. They make planning decisions at many levels and do not worry about consistency all the time. But both ideas have merit and are good for modeling different things. For example, people seem to plan dinners in a top-down manner.