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Expert surveys are frequently used in comparative politics to measure the ideological locations of political parties. However, it is possible that increasing the number of parties to place systematically biases results as experts try to fit more actors onto a common space. We test this possibility with an experiment embedded in an “expert” survey – with graduate students serving as our pool of experts to ensure an adequate sample size – by varying the number of parties to be placed in the United Kingdom and Germany. We find some tendency for the variance of Labor and SPD placements to diminish when more parties are present, and for SPD placements to move toward the center given more parties. However, we find no consistent evidence that the number of parties systematically affects mean or median party placements. Our results support the reliability of expert surveys as an indicator of party ideology.
