Statement of Criterion
Adding a preference to a ballot must not decrease the probability of election of any candidate ranked above the new preference.
The reasoning behind this criterion is that the voter should feel free to vote his complete ranking of the candidates, without fear that he is "giving away" information about his lower choices that the method may use against him.
Later-no-harm is satisfied by First-Preference Plurality, Instant Runoff Voting, Minmax(pairwise opposition), Douglas Woodall's Descending Solid Coalitions method, and Random Ballot. It is failed by virtually everything else.