Awkward: Part of Speech

imskpwr wrote:OG 13 Q 134:
"Recently implemented ""shift-work equations"" based on studies of the human sleep cycle have reduced sickness, sleeping on the job, and fatigue among shift workers while raising production efficiency in various industries."

Suddenly a thought came in my mind while reading this OG problem correct answer.
Which part of speech is "while" here. It has to be a Conjunction and not a Preposition.

So how can "raising" will go with "have reduced" verb?

Generally, we say

I am doing X while eating.
it means,
I am doing X while I am eating.

GMATGuruNY wrote:

imskpwr wrote:OG 13 Q 134:
"Recently implemented ""shift-work equations"" based on studies of the human sleep cycle have reduced sickness, sleeping on the job, and fatigue among shift workers while raising production efficiency in various industries."

Suddenly a thought came in my mind while reading this OG problem correct answer.
Which part of speech is "while" here. It has to be a Conjunction and not a Preposition.

So how can "raising" will go with "have reduced" verb?

Generally, we say

I am doing X while eating.
it means,
I am doing X while I am eating.

while + VERbing is an example of a VERBLESS clause: a dependent clause in which the subject and verb are omitted.
Generally, the implied verb will be in the SAME TENSE as the verb in the main clause.

John ate dinner while watching TV.
Conveyed meaning:
John ate dinner while {he was} watching TV.

Every night, John eats dinner while watching TV.
Conveyed meaning:
Every night, John eats dinner while [he is] watching TV.

SC134 in the OG13:
Recently implemented "shift-work equations" have reduced sickness while raising production efficiency.
Conveyed meaning:
Recently implemented "shift-work equations" have reduced sickness while [they have been] raising production efficiency.