1. Countries where the included studies took place.
| Country | Number of studies |
| Australia | 1 |
| Canada | 2 |
| Denmark | 1 |
| France | 1 |
| Norway | 1 |
| UK | 53 |
| USA | 10 |
| Multinational | 2 (one involving UK and Ireland and one involving the USA and Canada). |
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