I could be wrong here, but my understanding of channeling is that the ability to learn shows even before the girl "sparks" (see Egwene who was recognized as a potential channeler by Moiraine even though she did not reach her "change" yet).

If that is the case, a girl who fails the sul'dam test needs not be retested. She will never spark. Only the ones who test positive need to keep retesting. The sul'dam know that, and take that as part of the training.

In some way, as cruel as the Seanchan system is, at least it results in a lower death rate, since there is no chance of any girl dying of channeling because she was not found and trained before her change.

The sul'dam candidates are brainwashed from the outset to believe that they are saving the damane from a horrible fate, so if one of them tests positive and is made damane, she is probably relieved that she is "safe" from hurting herself and others...

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