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| Country | No. of studies |
|---|---|
| Argentina | 2 |
| Australia | 5 |
| Brazil | 1 |
| Canada | 4 |
| China/Hong Kong | 1 |
| Colombia | 2 |
| France | 6 |
| Germany | 5 |
| Hungary | 1 |
| India | 5 |
| Iran | 1 |
| Italy | 3 |
| Netherlands | 1 |
| Norway | 2 |
| Peru | 1 |
| Poland | 2 |
| Portugal | 1 |
| Romania | 3 |
| Russia | 1 |
| Singapore | 1 |
| Slovakia | 1 |
| South Korea | 5 |
| Spain | 10 |
| Tenerife | 1 |
| Thai | 2 |
| Turkey | 3 |
| UK | 2 |
| USA | 15 |
