I would like to express the fact that I don't have confidence in what a scientist has told me concerning an experiment he has led. Can I say that "his experiments are doubtful"?
Doubtful may be used in several ways:
- Something that is causing doubt;
- A person who is unsure or expresses doubt;
So, you can use doubtful in both contexts. If you wish to be most precise, finding a synonym is easy:
His experiments are questionable (ambiguous, problematic);
His experiments are not convincing;
I'm skeptical about his experiments;
You might want to consider the word dubious for expressing your sentiments:
His experiments are dubious.
According to Macmillan, the word means:
dubious (adj.) not sure about the truth or quality of something: I'm dubious about his ability to do the job.
marked by or causing doubt ⇒ a dubious reply
of doubtful quality; untrustworthy ⇒ a dubious reputation
Indeed, the word is sometimes used to describe experiments:
In a dubious experiment, Leary had given psilocybin to hardened inmates in a state prison1
An incomplete or less than certain theory becomes a "model"; a dubious experiment becomes a "simulation", and so on.2
Remember, "A dubious experiment can only beget a dubious result." (Glaucon)
Technically speaking, you could probably justify referring to doubtful experiments where you mean you have doubts about any conclusions drawn from those experiments (for whatever reason). But it doesn't sound very natural to me, and I think it's probably not a good idea.
Ordinarily, people are "doubtful" when they are "full of doubts". By extension, we often say some specific statement of fact is "doubtful", meaning we doubt (don't believe) that it's true. You might stretch that a little further and say that someone's argument is "doubtful", to mean the line of reasoning is flawed - but most people would simply assume you meant you doubted the conclusion was true, so it's not a very meaningful distinction.
When it comes to experiments, that distinction between the investigative process and the final conclusion suddenly becomes much more important. I haven't looked into the matter too closely, but I note that on this Wikipedia page it says "Contemporary knowledge concerning the manner in which the human body reacts to freezing is based almost exclusively on ... Nazi experiments".
What those Nazi "doctors" did is an extreme instance of what people normally mean when they refer to doubtful or dubious experiments. It's not that we question the truth of whatever the dubious experiment demonstrates - it's the morality of performing the experiment at all that we have "doubts" about.
I would advise OP to say he thinks the scientist's experiment is flawed, inconclusive, questionable, poorly designed, inadequately documented, or whatever other specific shortcoming he finds. An experiment itself isn't something that can be true or untrue, but it can certainly be [of] doubtful [morality], and that's what I think many if not most people would understand if you described it using the word.
Are you doubtful of his experiments, or just about results?
To be doubtful of the experiments suggests that you think his methods, or perhaps his presuppositions are faulty.
If you are doubtful only of the results, or the significance of the results, that's quite a separate matter.
For this reason, I would avoid saying "I am doubtful of his experiments" because it is ambiguous, and leaves the reader/listener wondering what you really doubt. Be more explicit.
I am doubtful of his conclusions.
I am doubtful of the methods used in his experiments.