It should be phrased as "........ have led the researchers to conclude that the type of treatment administered is suitable for the group"
Just how long did they scratch their heads as they poured over the statistics, with much agonizing over a period of time, with much disagreement and debate perhaps? - till finally a consensus: they agreed on their conclusion.
Present Perfect is not appropriate. The time span, from the statistical test results being spat out of the computer and someone noting the statistical significance levels, is irrelevant. It is the inference they drew from the results which is important: the results led them to conclude...
I would also suggest to Soup that he notes carefully the use of the definite article in the phrase "for
the group" and its bearing and implications for the type of research performed. Hence, I doubt whether the author was consciously employing the Historical Present - just the plain old simple type.
Re: Mixed Tenses in One Sentence 
Linear Mode
