ppauper wrote:

and
if you feed one but not the other, that is permitted

Indeed; a judge would not sentence you.a good one, that is.

Should I use a comma in the below sentence?
"Factor X reduces the value of a and b [to commar or not to commar] while increasing the value of c."

No, it reads better straight through with no pause.

P.S. "While" is a time-related word implying factor X is affecting a dynamic system over time. If Factor X is a logical condition (e.g., the first letter of a person's name) rather than a temporal property, then "whereas it increases the value of c" might be more correct. And if this a correlation (not a proven cause) then that affects the wording, too. Also is it "factor X" or high values of factor X that are associated with or cause conditional differences or dynamic changes in the other variables?

katastrofa wrote:

Should I use a comma in the below sentence?
"Factor X reduces the value of a and b [to commar or not to commar] while increasing the value of c."

Comma won't help; it does not remove the ambiguity. ppauper is correct.
A good test is: how would a programmer implement this text in C?

ppauper wrote:

Traden4Alpha wrote:

]
P.S.  "While" is a time-related word implying factor X is affecting a dynamic system over time.  

this statement is inaccurate because "while" as a conjunction has more than one meaning.

conjunction: while
1. during the time that; at the same time as.
"nothing much changed while he was away"
2. whereas (indicating a contrast).
"one person wants out, while the other wants the relationship to continue"

"while" is the correct word to use here.

However, a stickler for grammar might take exception to the use of a "dangling participle."
Thinking back many years to my PhD defense, I remember being savaged by one of the examiners (who was a very senior person in my field) for having several dangling participles in my dissertation, and I suspect he would rather wrote "while it increases"

I think I've been attacked on similar grounds (but I'm distrustful since the attacker yields some terrible errors themselves, e.g. "differ" -> "differentiate" or "alternative" -> "alternate" - aargh!). I am aware of the shortcomings and foregoings of my English grammar, but I cannot see how the participle construction in my sentence contains any ambiguity - actually, the whole subordinate clause with a repeated subject seems to me less efficient and slightly confusing. Is this purism or rather some snobbery? Similarly, the passive voice - in some scientific journals it's practically illegal to use it - ?!
Thanks guys. Definitely no comma and I will sleep on the above for a while.

Is this for an article? 
A good trick is to break the sentence into two uncoupled sentences and use active voice.

 I don't know what X,a,b and c are but I would define them like in maths books and then draw conclusions in such a way that no lawyer can whinge about them

"Factor X reduces the value of a and b [to commar or not to commar] while increasing the value of c."

??
Factor X is a monotonic decreasing as a function of a and b while it is monotonic increasing in c.

Maybe make the English as close to maths as possible. Sumphing like that.

ppauper wrote:

Perhaps,

Factor X reduces the value of a and b and increases the value of c.

or

Factor X reduces the value of a and b but increases the value of c.

I like the first one unless reader must watch out ACHTUNG for the case of to warn the reader. I don't know the context.
Factor X reduces the value of a and b; however, it increases the value of c.

I really like ppauper's "Factor X reduces the value of a and b but increases the value of c."
Thank you! You are my little angels <3