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Can, in some instances, conclude and decide be used just about interchangeably as far as AmEng goes?

Please, consider the following examples:

The committee concluded on a plan of action.

The committee decided on a plan of action.

-and-

We decided to postpone our vacation for a week.

We concluded to postpone our vacation for a week.

-and-

We found some stale bread, which we decided to eat.

We found some stale bread, which we concluded to eat.

-and-

He decided that dinner would be at 7 o'clock, and asked guests to arrive at 6.

He concluded that dinner would be at 7 o'clock, and asked guests to arrive at 6.

DECIDE

to infer on the basis of evidence : conclude they decided that he was right.

M-W

To make a judgment or determine a preference; come to a conclusion decide on where to go.

Random House

CONCLUDE

To reach as a logically necessary end by reasoning : infer on the basis of evidence *concluded that her argument was sound.

To make a decision about : decide concluded he would wait a little longer

M-W

To decide, determine, or resolve: He concluded that he would go no matter what the weather.

To arrive at an opinion or judgment; come to a decision; decide: The jury concluded to set the accused free.

Random House

To decide; determine.

Collins American English Dictionary

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2  
Normally you would not "conclude" to postpone something, since "conclude" implies a final decision, whereas "decide" leave wiggle room. – Hot Licks Mar 7 at 19:11
2  
I have never run into "concluded to". "Concluded that", yes. "Concluded to", no. – MετάEd Mar 7 at 20:19
    
"I concluded that the defendant was guilty, the jury decided he is innocent." – MonkeyZeus Mar 7 at 20:55
1  
FWIW Neither "conclude to" or "conclude on" are British English. oxforddictionaries.com/definition/english/conclude gives "conclude to" as "US, dated". – alephzero Mar 7 at 22:36
up vote 5 down vote accepted

Conclude suggests that a view or course of action has been been determined based on information or a process of reasoning.

Decide suggests that a choice has been made. It says nothing about the process used in reaching that choice.

He decided to have peanut butter cake, even though he was highly allergic.

After reviewing the patient's test results and the scarlet rash all over his body, the doctor concluded that the patient was allergic to peanuts.

Of course there is another sense of conclude having to do with ending a process but nothing to do with the determinative process.

He concluded the meeting with none of the key issues resolved.

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To decide involves making a choice, whereas to conclude generally implies a factual accuracy to your knowledge/conclusion.


Wrong usage

  • The committee concluded on a plan of action.
  • We concluded to postpone our vacation for a week.
  • We found some stale bread, which we concluded to eat.

Not okay, you conclude that, or in other words, you conclude propositions.

For example, "The committee concluded plan A was best" is fine becase "plan A is best" is a proposition (i.e. is true or false). Likewise, "We concluded postponing our vacation was the best option" because "Posponing our vacation is the best option" is a proposition.

I have never seen conclude followed by a preposition other than that.

Correct

  • The committee decided on a plan of action.
  • We decided to postpone our vacation for a week.
  • We found some stale bread, which we decided to eat.
  • He decided that dinner would be at 7 o'clock, and asked guests to arrive at 6.

These are fine, choices were made.

He concluded that dinner would be at 7 o'clock, and asked guests to arrive at 6.

Good. This one is also fine, but because of the "would be" construction, not because conclude and decide are interchangeable. It also implies that there was some preceding thought process.

After weighing how long the turkey would take to cook, he concluded dinner would be at 7 o'clock.


So, to answer your question, they are not just about interchangeable.

You can't just slot conclude into where decide went,

However, you can use them contextually in the same sense, see the alternate sentences I proposed for the committee and vacation in examples (For the bread example, I would say "which we concluded was safe to eat."), but that requires some changes in syntax. They are not straight-up

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Also, conclude tends to imply that the decision lies elswhere, as in, "there were conflicting messages about dinner, but the more credible evidence pointed to 7:00, so he concluded that it would start then." If the person denoted by "he" is actually determining when dinner will be, then "decided" is a much better choice. – phoog Mar 8 at 1:41
    
The "he" in the sentence is definitely the planner, see "asked guests to arrive." I would agree "decide" is better in speech, and "conclude" would be fine in writing, but it is rather affected. – Azor-Ahai Mar 8 at 1:47
1  
Right. The fact that "he" is definitely the planner leads me to, er, conclude that "conclude" is not correct in this case, though I wouldn't rule out some context that would make me change my mind about that. – phoog Mar 8 at 2:29
    
I attempted to provide such a context. I'll edit to make that part more clear. – Azor-Ahai Mar 8 at 3:20

I would say the first example is interchangeable. The second example is somewhat interchangeable, depending on the context.

The third and fourth examples are not interchangeable.

Decide is the default word choice for anything involving inferring, making judgments or otherwise arrive at some state of mind or action.

Conclude is a narrow meaning of decide, it is based on a chain of logical or evidentiary reasoning. Conclude also implies a terminal sense, coming at the end of some process of consideration and reasoning. Decide can occur spontaneously, in isolation without regard to any external considerations.

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2  
Are you a native speaker of American English? I am, and to me, "concluded on a plan of action" and "concluded to postpone our vacation" sound like the utterances of a non-native speaker, which certainly means the first two examples are not interchangeable. – phoog Mar 8 at 1:35

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