From whom and on what basis did the children receive their life assignments in The Giver?

Like many of the important placements of its citizens in
Lois Lowry's novel, The Giver, the Assignments for the Ceremony of
Twelve was given by the Committee of Elders. The Committee of Elders also presided over
the Matching of Spouses and the Naming and Placing of new children. The Assignments were
based on several factors, including disposition, energy level, intelligence and
interests. These factors had to interact perfectly with the Assignments (or spouses) and
they had to achieve a balance. Some of the Elders' Assignments proved to be surprises,
especially in Jonas' case.