In recent research, sequences of hypotheses observed during problem-solving have been categorized according to six hypothesis sampling systems that vary in efficiency. Three systems were characterized as strategies (focus, dimension check, hypothesis check) because they always lead eventually to solution. The remainder were called stereotypes (stimulus preference, position alternate, position perseverate) because they involve repeated manifestation of the same hypothesis (despite disconfirmation) and never lead to solution. Preliminary research revealed some striking developmental differences: Kindergarten children showed stereotypes in 90% of their problems; elementary-school children showed mostly dimension- and hypothesis-checking (65 to 80%) and little focusing (0 to 10%); college students showed focusing in half their problems and no stereotypes. These differences were compared to Piagetian stages of cognitive development. (Author/MV)