Never Lie.....but
You can never lie or make a knowingly misleading statement. Not only does that damage your reputation, it puts you in a legally precarious situation from both a civil and a criminal perspective. I have seen folks pay financially from completely unexpected quarters (e.g. a false statement, later proven as your lie as they subpoena your emails, causes some folks to be laid off) and criminally as well (e.g. fraud, SOX violations).
Lying also tells your sponsor you can be bought, so they will never trust you again, knowing that you can be bought. There is no honor among liars.
As to your reputation, those who suspect you've lied will think less of you and those who don't will know you are an incompetent.
On the other hand, it is not unethical to withold or not give an opinion or support a statement. This is common practice. If you don't want to support a negative or adverse conclusion you can say "I don't have enough concrete data or definitive proof that....". This will give you some time to prepare your client for an adverse outcome or to creatively change the environment/measurement/situation so as to not make it too painful for the client. You can't wait too long to admit the answer as well, because you will then be seen as unable to provide timely advice and counsel. Hedge, use the lack of data as an excuse, then work like hell in the background to fix it if you can. If not, then work with your client on a time and place to accept the inevitable, together.
Another possibly ethical (many times it is not, so watch it) is to not bring up the situation. Sometimes that works. I've been in meetings where there was a time constraint and I intentionally dragged out the discussion on another matter to be handled first or not covered the problematic issue and it was forgotten. By the time the meeting on that issue is called again, the issue has been fixed or has become irrelevant.
Be careful out there. Don't become a victim for a fistful of dollars.
