There have always been 'clashes' between AmEng supporters and BrEng lovers. What language to learn? American or British? As for me I prefer British to American.
Now, let's have a closer look at both of theses phrases: 'As a whole ' and 'On the whole'. A friend of mine said that Americans do not differ one phrase from the other. I'm afraid I can't go along with Americans on that score. Well, both of these idiomatic prepositional phrases are synonymous. You can find loads of similar synonyms for both of them. For instance: generally basically overall commonly largely mainly mostly predominantly principally substantially altogether chiefly customarily effectively habitually usually normally ordinarily regularly virtually by and large in general all in all as a rule in the main all things considered as a general rule for the most part generally speaking to all intents and purposes almost always as a whole in the general run of things more often than not on average on balance taking e