Answer:
matched-pair assignment
Explanation:
This study has two treatments. This researcher is using a matched pair from what we can see in the question. Matched pairs are used in the case of two treatment groups. The participants or subjects are grouped in twos. Among the two, she assigns different treatment to them. We can see that this researcher picks two people (pairs) from those that texts the most to those that texts the least. From each pair, one is a control treatment and the other is in the texting group.