Assessing the U.S. Senate Vote on the Corporate Average Fuel Economy (CAFE) Standard

Figures & data

Figure 1 Back-to-back stemplots of contribution amounts for the two parties.

Table 1. Descriptive statistics of contribution amounts by party.

Variable:	Amount
Party	Count	Mean	Minimum	Q1	Median	Q3	Maximum
Democrat	50	10025	0	1000	4375	14250	133250
Republican	49	17783	250	9250	15000	25000	48939

Table 2. Descriptive statistics of log-transformed contribution amounts by party.

Variable:	log(10A+1)
Party	Count	Mean	Minimum	Q1	Median	Q3	Maximum
Democrat	50	4.124	0.000	4.000	4.641	5.154	6.125
Republican	49	5.1318	3.3981	4.9660	5.1761	5.3979	5.6897

Table 3. Logistic regression results for the full model.

					Odds	95% CI
Predictor	Coef	SE Coef	Z	P	Ratio	Lower	Upper
Constant	−3.20274	1.64536	−1.95	0.052
REP	−8.04599	6.46223	−1.25	0.213	0.00	0.00	101.49
log(10A+1)	0.618414	0.350380	1.76	0.078	1.86	0.93	3.69
REP*log(10A+1)	2.01934	1.30583	1.55	0.122	7.53	0.58	97.39

Measures of Association: (Between the Response Variable and Predicted Probabilities)
Pairs	Number	Percent	Summary Measures
Concordant	1980	86.3	Somers' D	0.73
Discordant	301	13.1	Goodman-Kruskal Gamma	0.74
Ties	13	0.6	Kendall's Tau-a	0.35
Total	2294	100.0

Table 4. Logistic regression results for the reduced model.

					Odds	95% CI
Predictor	Coef	SE Coef	Z	P	Ratio	Lower	Upper
Constant	−4.51004	2.00965	−2.24	0.025
REP	1.91463	0.558377	3.43	0.001	6.78	2.27	20.27
log(10A+1)	0.898979	0.423062	2.12	0.034	2.46	1.07	5.63

Measures of Association: (Between the Response Variable and Predicted Probabilities)
Pairs	Number	Percent	Summary Measures
Concordant	1964	85.6	Somers' D	0.72
Discordant	318	13.9	Goodman-Kruskal Gamma	0.72
Ties	12	0.5	Kendall's Tau-a	0.34
Total	2294	100.0

Figure 2 A graphical comparison of the competing models must take into account the range of explanatory values — here displayed with boxplots.

Figure 3. Dotplots of contribution amounts by party, and the fitted logistic regression of the probability of a YES on party and amount contributed. Dotted lines are used for extrapolated values for the regression (six Democrats received no contribution; they are indicated by a solid dot). The vertical line at the $8000 contribution amount marks Vermont's James Jeffords, the sole independent in the Senate.

Table

Columns
1 – 18	Name (Last, First)
20–21	Two-character abbreviation for state
23	Party affiliation (R = Republican, D = Democrat, I = Independent)
25 – 27	Vote on CAFE standard (YES, NO)
30 – 34	Amount, lifetime contributions from auto manufacturers