Why this way?

When choosing a computer, consider these three factors: speed, memory and cost.

... consider these three factors:  the speed factor, the memory factor, and the cost factor.

Each of the three is a noun used as an adjective.  Nouns used as adjectives, that is, used to form compound nouns, are almost always in the singular.  The final version is simply an abbreviated form.

CJ

Alternate answer:

When choosing a computer, consider these three factors: speed, memory and cost.

... consider this:  Speed is a factor.  Memory is a factor.  Cost is a factor.
Or:
... consider this:  The speed is a factor.  The memory is a factor.  The cost is a factor.

All three can be used as noncountables (regardless of what your dictionary says).

CJ

Thanks, CJ.  That's beautiful!

In believer's second example, would you consider "approaches, methods, and contents" incorrect or just awkward?

Regards,   - A.

CalifJim wrote:

When choosing a computer, consider these three factors: speed, memory and cost. ... consider these three factors:  the speed factor, the memory factor, and the cost factor. Each of the three is a noun used as an adjective.  Nouns used as adjectives, that is, used to form compound nouns, are almost always in the singular.  The final version is simply an abbreviated form. Thank you, CalifJim. Would you say forming compound nouns like you did with those three words without quotation marks make them look awkward if not for those three words but for others? Please look below for further ideas on my point. Could I apply your line of reasoning to cases like this, including the example I have used in my original query post? More critical to the learning environment than approach, method, or content, are ... Applying your reasoning: More critical to the learning environment than the approach factor, the method factor, and the content factor are ... Now, to used these three words -- approach, method, and content -- as parts of compound nouns does look foreign at best, to me. They do not look familiar as the compound nouns as the previous three words -- speed, memory and cost. Let us deal with another case: I think one should not discriminate based on sex, race ... Could it be written like this using your reasoning? I think one should not discriminate based on the sex factor, race factor ... Now, although looks to be workable, like I said for the ones before this, forming compound nouns with the words 'sex' and 'race' doesn't look that familiar to me? Regardless of what is familair or not, should I apply your reasoning for similar cases? Then what criteria should I use to categorize something as similar? Thank you.             CJ Alternate answer: When choosing a computer, consider these three factors: speed, memory and cost. ... consider this:  Speed is a factor.  Memory is a factor.  Cost is a factor. Or: ... consider this:  The speed is a factor.  The memory is a factor.  The cost is a factor. All three can be used as noncountables (regardless of what your dictionary says). CJ

Could I apply your line of reasoning to cases like this, including the example I have used in my original query post?  No.  I don't think that my line of reasoning can be applied generally.  It just happened to work for that example.
...

Now, to used these three words -- approach, method, and content -- as parts of compound nouns does look foreign at best, to me. They do not look familiar as the compound nouns as the previous three words -- speed, memory and cost  My reasoning was intended as an explanation of the first example, not of all examples you will ever encounter generally.

It seems to me that the use of nouns in a series is (partially) what allows the dropping of the articles, so maybe the paraphrases are irrelevant anyway.  Perhaps a series of nouns resembles a list, so we use the same kind of short forms that we use when listing things.

.... these factors:
1.  speed
2.  memory
3.  cost

.... than these items:
1.  approach
2.  method
3.  content

CJ