### abstract ###
in a series of three experiments  participants made inferences about which one of a pair of two objects scored higher on a criterion
the first experiment was designed to contrast the prediction of probabilistic mental model theory  CITATION  concerning sampling procedure with the hard-easy effect
the experiment failed to support the theory's prediction that a particular pair of randomly sampled item sets would differ in percentage correct  but the observation that german participants performed practically as well on comparisons between u s cities many of which they did not even recognize than on comparisons between german cities about which they knew much more ultimately led to the formulation of the recognition heuristic
experiment  NUMBER  was a second  this time successful  attempt to unconfound item difficulty and sampling procedure
in experiment  NUMBER   participants' knowledge and recognition of each city was elicited  and how often this could be used to make an inference was manipulated
choices were consistent with the recognition heuristic in about  NUMBER  percent  of the cases when it discriminated and people had no additional knowledge about the recognized city and in about  NUMBER  percent  when they had such knowledge
the frequency with which the heuristic could be used affected the percentage correct  mean confidence  and overconfidence as predicted
the size of the reference class  which was also manipulated  modified these effects in meaningful and theoretically important ways
### introduction ###
the history of science and technology repeatedly demonstrates that many laws are discovered and many inventions are made serendipitously  as a by-product when researchers are striving for something else
the recognition heuristic is just one example of this  it was formulated as a post-hoc explanation for a puzzling finding that was observed while attempting to test a specific prediction of the theory of probabilistic mental models  CITATION
while the other papers contained in this series of special issues and many of the references given therein illustrate how stimulating the formulation of the recognition heuristic was and how much research it has spurred  the present paper turns back the clock and reports three studies that were conducted in the late  NUMBER 's and early  NUMBER 's  CITATION
this paper is organized as follows  the first part provides the historical context that led to the formulation of the recognition heuristic
at the outset of this part  a brief summary of pmm theory is given
experiment  NUMBER  is then reported  which was conducted to address one of the criticisms of the theory  namely the confounding of sampling procedure and item difficulty
specifically  we compared over underconfidence in two item sets that were generated by the same sampling procedure but were nevertheless supposed to differ with respect to percentage correct
this attempt failed  yielding the counter-intuitive finding that german participants performed about the same when making comparisons between german cities as when making comparisons between u s cities
experiment  NUMBER  reports a second  and this time successful  attempt to unconfound item difficulty and sampling procedure in order to answer the question that motivated experiment  NUMBER 
the second part is also historical  it reports experiment  NUMBER   which provides  to the best of my knowledge  the first empirical test of the recognition heuristic
it was designed to find out whether the results obtained in experiment  NUMBER  could be explained by participants having used the recognition heuristic
in this experiment  the participants' knowledge and recognition of each city was elicited  and how often this could be used to make an inference was manipulated
we also manipulated the inclusion criterion and  in turn  the size of the reference class that the cities were drawn from when constructing the paired comparisons
