In one embodiment, a first set of digital data (e.g., an image) is tested for the presence of a certain feature (e.g., a certain face), yielding one of two outcomes (e.g., not-present, or present). If the testing yields the first outcome, no additional testing is performed. If, however, the testing yields the second outcome, further testing is performed to further check this outcome. Such further testing is performed on a second set of digital data that is based on, but different from, the first set of data. Only if the original testing and the further testing both yield the same second outcome is it treated as a valid result. A variety of other features and arrangements are also detailed.