These words relate to suspicion, indecision or a lack of clarity. Doubtful can function in all three of these areas.

• She was doubtful of his good intentions; Still doubtful about which plane to take; The outcome of his appeal is still doubtful .

The word can also be used as an indefinite way to impugn the worth or value of something: a man of doubtful habits; a book of doubtful merit. Dubious is closely related to this last use of doubtful , but it carries a greater air of veiled insinuation and, relating to morals, may have more sinister overtones: a dubious person. The phrase, a doubtful person, by contrast, would pertain to someone who did not know what to do. Similarly, an outcome that was doubtful would be one not vet decided: a dubious outcome would be an undesirable one. Referring to hesitation, the word may suggest less basis on which to choose, but this may be accompanied by a sense of greater suspicion, wariness or even danger: She was beginning to be dubious about the man’s claim to be a window-cleaner. Questionable can be used euphemistically in reference to immorality: a house where questionable activities were conducted. In other uses, it points to something genuinely open to doubt, analysis or criticism: a questionable foreign policy.

Uncertain can refer, most simply, to an inability to choose: uncertain about which dress to wear. The word also applies to a lack of clarity or evidence or to an unforeseeable outcome: an uncertain legend on the rusted sign; an uncertain dating of the new fossil discoveries; these uncertain times, in which there is little to be hoped for and much to be feared. Problematic emphasizes one aspect of uncertain , referring to something that cannot be definitely established by the available evidence: The ultimate origins of the Aztec and Mayan cultures are likely to remain problematic . The word can also pertain to results that are endangered or unclear because of an attendant pattern of complex causality: Multilateral approval on each item of a nuclear-control agreement tends to make disarmament problematic .

Both ambiguous and equivocal concentrate on a lack of clarity. Ambiguous can refer to either an intentional or unintentional failure to be precise or definite.

• He kept making ambiguous remarks instead of straightforward yes-or-no replies; In explaining his stand, he only made everything more ambiguous than it had been to begin with.

Equivocal stresses an intentional wish to remain unclear, the word can, in fact, suggest deliberate deception or the saying of one thing while meaning another.

• She kept putting him off half-heatedly with equivocal replies to his proposal.

At its strongest, the word suggests something that might be thought of as morally compromising: They advised their daughter against putting herself in equivocal situations when she was out on dates. But the word can also apply to what is merely uncertain, unprovable or of doubtful validity, even when dubious motives are absent: statistical factors that make even the most scrupulous public-opinion polls equivocal in their findings.