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These analyses are generally supported by qualitative evidence. Even though they
provide excessive information in regard to party discourse, they are useful mostly for in-
depth single-case analyses. They suffer from the lack of a systematically designed
method, and of a cross-comparable metric in assessing party ideology. In this regard, the
pragmatic issue is the limited availability of quantitative measures in assessing party
ideology.
A widely-used quantitative data source of party ideology is expert judgments on
the position of parties on the left-right scale or particular issue/policy scales. Country
experts are asked to estimate scores of political parties on a uni-dimensional left-right
spectrum. Then, averages of individual scores are calculated for each party. Although
expert scores are used at large to measure party positions, there are notable limitations
due to their calculation method and metric. Most importantly, expert scores do not
account for ideological change over time because expert surveys have been “administered
infrequently, in different formats, and only over the last fifteen years” (Gabel and Huber
2000, 94). Put differently, expert scores are not appropriate for measuring preemption
and accommodation because they are stable.
The Comparative Manifesto Project (CMP) is another method to calculate the
left-right scores of parties. It overcomes the stagnancy of the expert scores by extracting
the relative weight of issues that parties emphasize in their electoral manifestos over
legislative elections (Budge 1994, Budge, Robertson, and Hearl 1987, Budge et al. 2001).
Therefore, the CMP is the only appropriate dataset that provides systematic information
as to the existence and extent of spatial occupation of the right, or the spatial movement
to the right, if any.
Convention