- With both tests (ADF, Phillips-Perron), we would reject the null hypothesis of a unit root and conclude that the approval series is stationary.
- This makes sense because it is hard to imagine a bounded variable (0-100) having an infinitely exploding variance over time.
- Yet, as scholars have shown, the series does have some persistence as it trends upward or downward, suggesting that a fractionally integrated model might work best (Box-Steffensmeier & De Boef).
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