🔍 The Problem

Spatial models of political competition assume parties adjust their ideological positions in response to rivals, and spatial econometric techniques are widely used to model these interdependencies. These approaches typically treat party positions as perfectly measured covariates, but party ideology is actually latent and accompanied by measurable uncertainty. The implausible assumption of error-free measurements can distort empirical tests of theoretical expectations.

📊 How the Model Works

A novel model combines spatial econometrics with a Bayesian dynamic item response framework to account for measurement uncertainty while estimating strategic interdependence. Key features:

• Simultaneously estimates latent party ideological positions and the spatial dependencies among parties.
• Uses a Bayesian dynamic item response model to represent uncertainty in measured positions across time.
• Integrates those latent estimates into a spatial econometric structure so that interdependence parameters reflect measurement error.

🗳️ What Was Analyzed

• Recorded roll-call votes from all sixteen German state legislatures.
• Time span: 1988–2016.
• Analysis focuses on ideological mobility and spatial dependence among parties, with attention to party-family similarity.

🔑 Key Findings

• Parties display a notable degree of ideological mobility over the 1988–2016 period.
• Spatial dependencies among parties of the same party family are present but only moderate in magnitude.
• Accounting for measurement uncertainty changes substantive conclusions compared with models that assume perfectly measured covariates, sometimes producing qualitatively different estimates of interdependence.

⚠️ Why It Matters

• Measurement uncertainty in latent ideology matters for testing theories of strategic positioning: ignoring it can mislead inference about how and whether parties respond to rivals.
• The combined Bayesian dynamic IRT + spatial econometric approach provides a transparent path from theoretical expectations about interdependence to statistical estimates that respect uncertainty in measured positions.

Together, these results illustrate both the empirical importance of ideological mobility in German state politics and the methodological necessity of modeling measurement uncertainty when evaluating spatial theories of party competition.