In each model specification, the coefficient on the contractual indicator is negative and is significantly different from zero. Note that coefficients in survival models change the probability of an event, i.e. The danger rate, express it. Here, the event is defined as an agreement that ends. Therefore, a negative coefficient indicates a decrease in the likelihood that an agreement will be cancelled if it is concluded in the form of a treaty. The results imply that contracts take much longer than executive agreements and that the difference in sustainability is not the result of arbitrary factual conventions or a by-product of a decision-making process determined primarily by the Senate seat card. In an attempt to answer this question, scientists have made several hypotheses about the contemporary role of the treaty. These can be roughly classified into two categories: first, there are hypotheses that support the idea that treaties have no independent value as a political instrument. These reports typically explain the use of the treaty by motivations that are orthogonal to the reflection on the quality of the promise itself.

Second, there are assumptions that promises made as treaties differ qualitatively from those made as executive agreements and that the decision to use the treaty is determined by these considerations. As explained in the text, survival periods are continuous in nature, with international agreements being able to stop at any time. However, since survival periods are only measured once a year when the TIF is published, the data can be described as continuous data, aggregated by year. For truly continuous data where an event can occur at any time, the Cox proportional-hazard modelFootnote 117 has emerged as the preferred choice for researchers, footnote 118, as it is a semiparametric model based on a few assumptions. The popularity of this model comes from the fact that it can be appreciated without making parametric assumptions about the base hazard rate. For example, the researcher does not have to expect survivability to decline at a constant rate over time, exponentially, or by other predefined means. However, the Cox model estimates that there are no links in the data, meaning that no observation has exactly the same survival time. This is because links cannot occur when survival times are measured on a truly continuous scale. Researchers have developed several techniques that make the Cox model manageable even for fasteners. The most accurate approach is the “exact method” developed by veal and Prentice. Footnote 119 Intuitively, if exactly two subjects i and k survive periods, the alternative I survived longer than k and the alternative that k survived longer than i and chooses the one that is more likely.

Footnote 120 However, in data sets with many themes, periods and attachments, the exact method is not feasible because it is very computational. The Efron 121 method provides an approach to the exact method, which does not suffer from comparable resource restrictions, but is somewhat less accurate. Future research could address this restriction by sorting agreements at a more detailed level than can be relied on TIF data. A particularly promising approach could result from a detailed analysis of the content of the various agreements. Analysis of the text of treaties would allow researchers to distinguish, for example, between bilateral tax treaties for the avoidance of double taxation, often concluded in the form of treaties, and other types of tax treaties. The databases relevant to the conduct of such studies exist,footnote 105 but it has not been possible so far to read and categorize several thousand international agreements in a coherent manner. . . .

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