Answer:
Adverbial
Step-by-step explanation:
An adverbial phrase tells us how, when, where, and why something happens in a sentence. An adverb also serves to qualify an adjective, a verb, or another adverb.
In the sentence above, "where", has an adverbial connotation for it tells us the manner in which the research is performed- "half of the participants receive training and the other half do not." So from this definition, we can conclude that the sentence is adverbial.