Can "to winnow out" or "to screen out" mean "to extract or identify (something desirable)"?

Note: this answer has been heavily edited, so some of the comments below might now seem out of place or irrelevant.

It seems that the meaning of "winnow out" varies: "winnow out" can be used to mean "remove what you want" or "remove what you don't want". It's real meaning may be closer to "separate", the verb.

I suppose it's a form of separation which is done gradually, by taking things out. You could throw those things away, and just keep what's left, or you could put the things you took out in a pile, and keep them.

Whether or not this interpretation is correct, it seems to mean one thing to some people and the opposite thing to others, including many dictionaries! eg the Collins dictionary lists examples using both versions:

http://collinsdictionary.com/dictionary/english/winnow-out

"The committee will need to winnow out the nonsense and produce more practical proposals if it is to achieve results." (remove) and "Time has winnowed out certain of the essays as superior." (keep).

A lot of people might just have no idea at all what it means - it's not a common phrase.

This means that using "winnow out" is liable to cause confusion, and so I'd recommend against it.

When we "screen out" something we're trying to remove that thing, like someone trying to detect a signal would attempt to "screen out background noise".

So, neither of them are appropriate in this context: "winnow out" is too informal (in my opinion) and potentially confusing, and "screen out" is wrong for the reason stated above: it suggests that the organisation doesn't want to see the best candidates.

I'd suggest "identifies" (if it just wants to find out who they are and not necessarily do anything yet) or "selects" if it wants to actually choose them.

First, some definitions:

Winnow verb
1.1 Remove (chaff) from grain: women winnow the chaff from piles of unhusked rice
2 Remove (people or things) from a group until only the best ones are left: guidelines that would help winnow out those not fit to be soldiers - ODO

I've included definition 1.1 for winnow as it provides some insight into how the term is used.

Screen verb 3.3 (screen someone/thing out) Exclude someone or something after evaluation or investigation: anti-spam software can screen out large amounts of unwanted email - ODO

As commonly used, winnow is associated with the idea of removing unwanted portions (it's more nuanced, but this is the relevant part here). Naturally, what is left is the desired portion. The unwanted parts are discarded (thrown out, as it were). These discarded parts may be said to be winnowed out. The example provided in definition 2 is consistent with this - it's those not fit to be soldiers that are "winnowed out".

One may ask whether the desired elements can be said to be "winnowed in". However, as Ngram suggests, that phrase isn't idiomatic. The process of winnowing is one of removal, so it isn't natural to apply it directly to a process of retention.

Likewise, screen out is associated with the removal or exclusion of whatever has been screened out.

In answer to your question, then, neither winnow out nor screen out are normally used to refer to the retention of "something desirable". The goal of winnowing or screening, however, is to end up with that "something desirable". You can therefore say that after winnowing out (or screening out) the undesirable elements, the "desirable" elements remain.

It's possible to go further, to say that the group has been winnowed (note: not winnowed out) to the best remaining candidates, as in the following example:

In addition, the program first employed less expensive screening strategies, such as reviews of official records, reserving the more expensive medical exams until the pool of candidates had been winnowed to a more manageable level. - Boot Camps for Juvenile Offenders - by Blair B. Borque