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1) that (in "that of abot 334 who needed treatment") = the health condition?

2) 107 accessed it = 107 accessed the free ART clinic?

Context:

Before June 2005, the researchers report, the mortality rate among adults aged 15-59 years was 9.8 per 1000 person-years of observation. The researchers found that the probability of both men and women dying between these ages was about 43% and about 65% of deaths (229 of 352) were related to AIDS.

The data collected eight months after the opening of the free ART clinic indicated that of about 334 who needed treatment, 107 accessed it. Still, there was a 10% reduction in mortality in adults, and a 35% reduction in adults who were near the main road (a location where pre-ART mortality was greatest). When subgrouping by age, the researchers found that mortality rates in adults aged 60 years or more did not change.
Comments  
"That" is just a conjunction here, like in: «The day that the rains came», «That'll be the day that I'll die», or «I know that you are here»

You can decmpose that sentence as follows:
«The data (...) clinic indicated: Of about 334 who needed treatment, 107 accessed it.»
Thank you Ant_222, you are sharp eyed.
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Ant_222You can decmpose that sentence as follows:
You deconstruct a sentence. A dead body decomposes.
Actually, Ant_222 is using decompose in its first (transitive) definition! (Surprise!) (m-w.com)

Main Entry:de·com·pose Pronunciation: \ˌdē-kəm-ˈpōz\ Function:verb Etymology:French décomposer, from dé- de + composer to composeDate:1718
transitive verb 1 : to separate into constituent parts or elements or into simpler compounds <decompose water by electrolysis> <decompose a word into its base and affixes> 2 : rot intransitive verb : to break up into constituent parts by or as if by a chemical process : decay, rot <fruit decomposes>
I must admit that the first definition above is not the most commonly used, however.
Emotion: smile
CJ
The things you learn. I didn't even know it HAD a transitive sense. My apologies, Ant.
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GG: «The things you learn. I didn't even know it HAD a transitive sense. My apologies, Ant.»

No problem, GG. I didn't even know it had an intransitive meaning! Also, you reminded me of another word — "deconstruct", which I couldn't imagine applied to "sentence" even in dreams... I thought of its meaning like "to disassemble constructions" — cars, buildings, engines... but not sentences.