possibility vs probability

[ONI]

Dear teachers,

I'd like to know the difference between possibility and probability in modal verbs.
Possibility
He can win the race. (90% certain)
They could still be at school. (50% certain; it’s possible they are still at school.)
Tom may be studying in his room. (perhaps; 50% certain; it’s possible that he’s studying.)
He might want some more food. (40% certain; perhaps he wants some more food.)
It is likely that he will arrive tonight.
He is likely to arrive tonight.

──

She could have been killed in the car crash. (Luckily, she wasn’t killed.)

He may have spoken to Jenny yesterday. (Perhaps he spoke to Jenny.)

He might have forgotten. (Perhaps he has forgotten.)

It was likely that he had arrived the day before.
He was likely to have arrived the day before.
Probability
They will be home soon. (100% certain; prediction)
Greg should win easily. (90% certain; future only; he’ll win easily.)
They ought to be home by now. (90% certain; they will probably be home.)

____

He should have received his prize by now. (He has probably received it by now.)
They ought to have arrived an hour ago. (They have probably arrived)


I understand the above table, but I really don't understand the difference.

Thank you!

[albertino]

Possible, probable (adj) and likely (adj & adv) all denote "likelihood" in meaning.
However, there are some subtleties between them.

First of all, "possible" emphases the likelihood in an objective manner, but it always implies a sense of "the likelihood is minimal". For example, I would say that while failure is possible, everything points towards success.

Next, "probable", according to its etymology, means verifiable, and thereby extends to mean "substantiated, reasonable, and credible". In contrast to "possible", it may well indicate a strong sense of likelihood.
For example: (1) It's possible, thought not probable, that he would accept the offer. (2) The probable cause of his death has been diagnosed as avian flu.

Finally, "likely", on the surface of it, is similar to "probable". They are sometimes interchangeable with not much difference in meaning.
For example: It is likely (probable) that we shall meet with some opposition.

The difference between "likely" and "probable", if you like, will be that the former sets great store by the judement that is arrived at from some superficial signs, whereas the latter is based on the truth that is revealed after considering the plus side and negative side of a situation.
For example, (1) The likely outcome of the contest varies from moment to moment(Either A or B will win as both of them are equal in strength and skills. The outcome is unclear). (2) On the probable outcome of the contest, opinions of the audience are unanimous(Obviously, A is stronger than B). Hope that helps a bit.
(Not a teacher)

[riverkid]

Originally Posted by ONI

Dear teachers,

I'd like to know the difference between possibility and probability in modal verbs.
Possibility
He can win the race. (90% certain)

They could still be at school. (50% certain; it’s possible they are still at school.)

Neither 'can' or 'could' state any particular level of certainty/probability, Oni. They both say only "It's possible that ..."

Tom may be studying in his room. (perhaps; 50% certain; it’s possible that he’s studying.)
He might want some more food. (40% certain; perhaps he wants some more food.)
It is likely that he will arrive tonight.
He is likely to arrive tonight.

──

She could have been killed in the car crash. (Luckily, she wasn’t killed.)

He may have spoken to Jenny yesterday. (Perhaps he spoke to Jenny.)

He might have forgotten. (Perhaps he has forgotten.)

It was likely that he had arrived the day before.
He was likely to have arrived the day before.
Probability
They will be home soon. (100% certain; prediction)
Greg should win easily. (90% certain; future only; he’ll win easily.)
They ought to be home by now. (90% certain; they will probably be home.)

____

He should have received his prize by now. (He has probably received it by now.)
They ought to have arrived an hour ago. (They have probably arrived)


I understand the above table, but I really don't understand the difference.

Thank you!

Take any statement/collocation/structure that denotes 100% certainty and the modals and semi-modals modify it roughly as follows. Let's use your last sentence:

100% They have arrived.

90-99% They almost certainly have arrived. // They must have arrived.

['must' isn't always possible here. Certain conditions must be met to use 'must']

76-89% They very likely have arrived.

51-75% They likely/probably have arrived.

['should' also fits in this same range, 51-89%]

26-50% They may have arrived.

1-25% They might have arrived.

These numbers are not cast in stone. They are a rough representation of highly elastic words. What they do is illustrate the comparative differences between the modal/semi-modals and give the range that they cover.

This same range cover any situation, past, present or future. Obviously changes are made to denote the syntactic differences between, say, past and present.


100% They will/are going to arrive at ... .

90-99% They almost certainly will arrive at ... . // ?They must arrive at ... .?

['must' isn't always possible here. Certain conditions must be met to use 'must']

76-89% They very likely will arrive at ... .

51-75% They likely/probably will arrive at ... .

['should' also fits in this same range, 51-89%. But like 'must', it can't always be used.]

26-50% They may arrive at ... .

1-25% They might arrive at ... .