In general usage, effectiveness and efficacy have the same meaning. In medical jargon there is a subtle, but important difference. From Wikipedia: In medicine, effectiveness relates to how well a treatment works in practice, as opposed to efficacy, which measures how well it works in clinical trials or laboratory studies.

I feel that there is a subtle difference in meaning between 'efficacy' and 'efficiency', but I couldn't find any authoritative sources that could help me confirm or refute this. Is there any differ...

I have trouble understanding the difference in the meaning of "effective" and "efficient". what is the difference between "efficient learning" and "effective learning"?

The choice as to which to use comes down to the degree of formality of your text, and possibly also how much space you have available (e.g. when putting together a PowerPoint slide). In general, using & implies a much more informal tone than and. You will never be criticized for using and, whereas you run the risk of disapproval if you use & in anything but informal notes, tweets and the like.

Do you live anywhere near him? Do you live somewhere near him? Is there any difference between these two sentences?

What is the difference in meaning between the above words? Are they interchangable? I have seen several instances where efficacy has been used where it seems that efficiency is the correct term. Pl...

Some of these combinations sound better than others, e.g. "acoustical energy" sounds wrong to me, but that may be purely subjective. Is there any difference in meaning between the words? Or might it just be a UK/US English difference?

Thank you Tony. Do you mean that only combination of is right. So the expressions of combination between and combination among are incorrect.

completely different. * www.*urbandictionary A phrase used to describe a stark difference between two things. Similar to day and night, except it implies an improvement of the situation rather than a deterioration. So my question: Is the usage of ‘night and day” in the meaning of “completely different” popular, or still on the sideline?

In American English usage, shall is used only in legal formulae, archaic language like prayers and hymns, and in the invitation formula Shall we VP? and the offer formula Shall I VP?, both with first person subject. Other than that, it's not used at all. Of course, as a modal -- any modal -- there are many, many possible "meanings" of shall, and even more of shall not, depending on how precise ...