In the phrase:
It may be best for you to decide on your target audience.
would replacing "decide" with "determine" be more correct, and why?
If you "determine your target audience", then you may be deciding on your target audience, or you may be analyzing your content to figure out who your target audience is likely to be.
Just to highlight it a different way:
There's a definite difference there which is important in the context of presentation and salesmanship that I assume is the background to the question. In such contexts it's definitely important to be flexible about who you are targeting and not decide on a target audience too quickly.
There would be contexts where determine/decide are interchangeable, but not in OP's example.
It may be best for you to decide on your target audience.
...means you are going make the (possibly reasoned, but essentially free) choice of what target audience seems best to you.
It may be best for you to determine your target audience.
...implies that your target audience already exists, and that your task is to identify it - by whatever means you like, but you don't get to choose the target audience. You find them.
Decide and determine are very similar in meaning. In some cases they can be used interchangeably, but in the example you cited I think the choice changes the meaning of the sentence subtly:
Determine the target audience.
Decide on the target audience.
They both mean generally the same thing, using the following definitions from M-W:
Determine: to find out or come to a decision about by investigation, reasoning, or calculation
Decide on: to make a choice or judgment
There is a very subtle difference to me, which is that determine implies that some additional investigation is needed in order to make the decision.
Determine on your target audience.
This is technically correct. Determine does have an intransitive form, but it sounds really awkward to me. A google ngram search shows that this form has been trailing off sharply since the 1800's in favor of decide on.
Decide your target audience.
Grammatically this is correct but it has a different meaning. It sounds like you're trying to get your audience to make a decision.
Decide has the connotation that a conscious thought process was spent on coming to a particular conclusion.
I haven't yet decided what my target audience is; I'm still going through all the possibilities.
Determine has the connotation that a conclusion has been made precise without much implication about the process.
I haven't yet determined what my target audience is. I haven't gotten the stats yet.
Of course, these meanings are pretty close and so interchanging them won't terribly affect the implications.
"Determine your target" certainly is more svelte than "Decide on your target". Of course there are many appropriate ways to use decide on, but also a good many that sound wrong to me. Note, also consider focus on and study in place of determine.
The ngrams shown below compares decide on, decide about, decide to; I include this to show that my bias against decide on may have a rational basis.
It shows that printed usage of decide to overtook that of decide on ca. 1900.
Regarding determine or determine on, an ngrams mentioned in another answer shows that determine on has little usage, and that is as it should be; an on after determine is superfluous, perhaps incorrect.
I'm going to go with a "bell curve" for my answer. Assuming the author is intending to address, say, 20+ members of the public, he chooses a mean and standard deviation of audience characteristics for his publication. To that extent, he makes that particular decision, but there are a number of important events which precede and result from it.
Initially, he might make some observations about existing audiences in the domain or nearby domains of his subject matter, not dissimilar to doing market research. After he publishes, the existing "bells" change to one extent or another due to his audience's reactions, and via the grapevine. Assuming he publishes further, he then makes new observations of the audience, and makes another decision. The process is incremental in this way. Notably, his decision of audience traits might precede his publication by a significant time.
I have observed idiomatic usages of the terms in question I would interpret to be incorrect; however grounds for that assertion aren't strong. In general, it's possible for more than 50% of speakers to be wrong about the meaning of a word. Language is not entirely convention, but even accounting for consistency with stems, declensions, and etymologies, that number doesn't rise far.
In those usages, again, word choice regarding essentially other word choices, I got the impression that the speaker was or believed himself to be making a ruling for a large number of others; in particular, whether that group would include the matter in question as among the permitted usages of the word, or exclude it, even to the exclusion of communicating the physical quantities being described. It was almost as though the word choice was a guest list for a party.
I might be reading too much into the OP's question, this isn't a self-help forum, and I'm not a licensed clinical therapist. But, furthermore, I feel strongly that the above transposition of terms not only detracts from general language maintenance and language integrity, but could be harmful to the speaker himself, exacerbating already existing conflicts in his priorities, and obscuring the forest for the trees.
M/30/Outside Chicago/LAS bachelor and computer programmer
If I invent a new product, I would decide my audience, and sell to that audience. If I have to give a keynote address, I would determine my audience, and adjust my speech to appeal to that audience.
When you use the term Determine you would normally need to qualify it further, such as "determine your target audience's attitudes and behaviors" -- determine typically needs a 'what' (attitudes) and a 'who' (your audience). If you use Define it works as 'define your audience' and 'define your audience's attitudes' -- to be safe and simple, I would recommend using 'define'.
Determine = understand. Define = choose or qualify.