Reference equations are developed from population studies and the measurements that come from these studies almost always fall into what’s called a normal distribution (also known as a bell-shaped curve).Ī normal distribution has two important properties: the mean value and the standard deviation. The Z score is instead a description of how far a result is from the mean value and therefore emphasizes the probability that a result is normal or abnormal. The LLN primarily emphasizes only whether a result is normal or abnormal. The difference between the two however, is in the emphasis each places on how results are analyzed.
Both the Z score and the Lower Limit of Normal (LLN) come from the same roots and in that sense can be said to be saying much the same thing. The use of Z scores to report PFT results, both clinically and for research is occurring more and more frequently.
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