Table 5 summarizes TTB use by our 31 economies over 1995-2013. It also includes information on when the economy implemented its antidumping law, and when it initiated its first antidumping investigation. We choose the 1995-2013 period for a number of reasons. First, it is a period that our data most accurately captures the ‘stock’ of TTB policies in effect.<sup>50</sup> Second, 1995 initiated common rules for TTB use for all WTO members.<sup>51</sup> Third, by 1995 we observe a common external trade policy for customs union partners such as EU-Turkey and Argentina-Brazil and thus can more easily examine potential differences in TTB use between partners.

For interpretive purposes, consider the import coverage of the TTBs that the United States had in effect over 1995-2013. The first four columns of Table 5 reveal information on the cumulative share of imported products over which the United States imposed some sort of TTB policy during the period. The US imposed some TTB policy on 10.6 percent of all HS06 imported products at some point during 1995-2013. Among the four different TTB policies in use by the United States during this period, antidumping has been most prevalently applied (covering 9.0 percent of all products), followed by countervailing duties (5.1 percent), the global safeguard (2.8 percent), and the China-specific transitional safeguard (less than 0.1 percent). The fact that individual TTB policies for the US aggregate to more than 10.6 percent of total imports reflects both the _substitutability_ of these policy instruments - e.g., the United States has applied different TTB policies to the same products at different points in time - as well as the _redundancy_ of these policy instruments - e.g., the United States frequently applies two different TTB policies, such as an antidumping duty and a countervailing duty, to the same product and trading partner at the same time. The last columns indicate that, on average over 1995-2013, the US had 4.9 percent of imported products covered by an imposed TTB in any year, and the maximum coverage was at 6.8 percent in 2012. Finally, the mean share of imported products in a year that were subject to a new US TTB investigation - and that could potentially lead to new import restrictions - was 0.9 percent. The maximum share of imports subject to new TTB investigations was 3.9 percent of products in 2001, when the US initiated a wide-ranging safeguard investigation over steel.

Is there a pattern to which countries tend to utilize TTBs as import protection? To summarize Bown (2011b), which also provides more detail on the underlying methodology, using more recent data from the World Bank’s Temporary Trade Barriers Database (Bown, 2014a). Additional detail on the database is provided in the Data Appendix. Our analysis below is not comprehensive, however, and there are at least three other important aspects of TTB use that we mention briefly here. First, and as we describe elsewhere in this chapter, these policies are applied not only as ad valorem duties, but also frequently as specific duties, price undertakings, quotas, and tariff rate quotas. Second, our import coverage ratio measures do not address potentially significant variation in the restrictiveness of the policies; here we only note that even when these policies are imposed as ad valorem import duties, they are sometimes set at prohibitive levels of greater than 100, 500, or 800 percent. Third, while the ‘stock’ measures introduced below are affected by the duration of the imposed policy - recall that WTO rules for each of them are that they are supposed to be ‘temporary’ (i.e., duration of less than 3-5 years) - there are numerous examples of duties covering some products that have been imposed for 20 - 30 years or longer. The data required to explore each of these points empirically is readily available in Bown (2014a).

> 50For the US and EU especially, the early 1990s featured antidumping duties still in effect from the 1980s and earlier but for which we do not have the HS codes because they were imposed under different product classification schemes.

> 51Under the GATT, the rules for certain TTBs were different depending on whether a GATT member was a signatory to the plurilateral Antidumping Code and Subsidies Code.

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