Standard errors of raw scores decrease to zero, misleading the analyst into believing that zero and perfect scores imply exact knowledge of the location of examinees on the latent variable. Decisions can be made on an individual rather than group basis. Test Length and Reliability Old Rule 2. Longer tests are more reliable than shorter tests. New Rule 2.
Shorter tests can be more reliable than longer test.
No, as New Rules clarifies, the Spearman-Brown prophecy formula is not revoked. Provided everything stays the same, a longer test of the same sort of items is more reliable than a shorter test. But a longer test is not necessarily more reliable than a different, shorter test. But classicists couldn't do much with this knowledge, because everyone had to take the same test, and test content was fixed. Now there are item banks and computer-adaptive testing.