ECONOMY

keith krehbiel 237
Sum of squared roll rate errors
0 5 10 15
(House, Uniform)
25 50 75 100
Maximum range of status quo shock
Sum of squared roll rate errors
0 5 10 15
(House, Normal)
25 50 75 100
Maximum range of status quo shock
Sum of squared roll rate errors
0 5 10 15
(Senate, Uniform)
25 50 75 100
Maximum range of status quo shock
Sum of squared roll rate errors
0 5 10 15
(Senate, Normal)
25 50 75 100
Maximum range of status quo shock
Procedural cartel
Pivotal politics
Median voter
Fig. 12.5 Comparison of predicted roll rate errors using endogenous-status-quo
method
party rolls, and few such rolls occur. Meanwhile, the median voter and pivotal politics
theories systematically over-predict such rolls. 18
But while the cartel theory is uniformly successful in predicting majority party
roll rates, it is uniformly abysmal in predicting those for the minority party. Without
exception, it predicts far more minority party rolls than in fact occur. Indeed, all of the
errors summarized in column 2 are due to over-prediction and these are considerably
greater than any other theory’s errors in columns 1–6. Consequently, when roll types
are combined and the overall predictive accuracy of the theories is compared in
columns 7–9, the cartel theory is the poorest performer.
For most assumptions about the distribution of the status quo shock term, the
median voter theory performs best of the three models. This is always the case under
the assumption of approximately normally distributed shocks. The pivotal politics
theory, in contrast, generates the fewest errors under the assumption of high variance
of uniformly distributed shocks.
The mixed nature of the findings—and especially their clear dependence on the
assumptions about the error term of the status quo shock—has two implications.
First, the findings underscore the importance of employing an explicit framework
that can accommodate a wide range of assumptions about the status quo. Second,
¹⁸ When the House data in the first six rows of Table 12.3 are partitioned into Congresses in which a
given theory over- versus under-predicts, the pivot and the median voter theories never predict fewer
majority party rolls than occur. It is likewise rare in the Senate.