I present an average case analysis of propositional STRIPS planning. The analysis assumes that each possible precondition (likewise postcondition) is equally likely to appear within an operator. Under this assumption, I derive bounds for when it is highly likely that a planning instance can be efficiently solved, either by finding a plan or proving that no plan exists. Roughly, if planning instances have n conditions (ground atoms), g goals, and O(n g÷d) operators, then a simple, efficient algorithm can prove that no plan exists for at least 1 - d of the instances. If instances have W(n(ln g)(ln g/d)) operators, then a simple, efficient algorithm can find a plan for at least 1 - d of the instances. A similar result holds for plan modification, i.e., solving a planning instance that is close to another planning instance with a known plan. Thus it would appear that propositional STRIPS planning, a PSPACE-complete problem, is hard only for narrow parameter ranges, which complements previous average-case analyses for NP-complete problems. Future work is needed to narrow the gap between the bounds and to consider more realistic distributional assumptious and more sophisticated algorithms.