Complex sentence: They had to mark their first preference with 1, their second with 2, etc. The new method was first applied in Belgium, and when results showed the name-letter effect at work again, it was copied in the other countries.
Split into simpler sentences: They had to mark their first preference with 1, their second with 2, etc. The new method was first applied in Belgium. When results showed the name-letter effect at work again, it was copied in the other countries.