Based on English alphabetical order, three of the four letter-cluster pairs…

Concept

Represent each letter by its alphabetical position and compare the change from each letter in the first cluster to the corresponding letter in the second cluster. Pairs belong to the same pattern when both position-change values are identical.

Application

1. For IF−HE: I (9) becomes H (8), so the first change is −1; F (6) becomes E (5), so the second change is −1. The change vector is (−1, −1).

2. For PM−NN: P (16) becomes N (14), so the first change is −2; M (13) becomes N (14), so the second change is +1. The change vector is (−2, +1).

3. For ZW−YV: Z (26) becomes Y (25), so the first change is −1; W (23) becomes V (22), so the second change is −1. The change vector is (−1, −1).

4. For MJ−LI: M (13) becomes L (12), so the first change is −1; J (10) becomes I (9), so the second change is −1. The change vector is (−1, −1).

Contrast

• IF−HE uses the common (−1, −1) change.

• ZW−YV uses the common (−1, −1) change.

• MJ−LI uses the common (−1, −1) change.

• PM−NN uses (−2, +1), so its two letter changes differ from the common rule.

Cross-check

Applying one backward alphabet step to both letters gives IF→HE, ZW→YV, and MJ→LI. It does not give PM→NN.

Result

Therefore, PM−NN is the pair that does not belong to the group.