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Clinical Insights

Interpreting results with GLI-2012 using Z-score and LLN

In the four years that it took the Global Lung Function Initiative (GLI) to finish its mission, with the support of six large international respiratory societies, a collaborative network was established that spanned the world.

The network included clinicians, researchers, technicians, IT engineers and manufacturers. The objective was to derive reference equations for spirometry that covered as many ethnic groups as possible, and an age range from pre-school children to old age. This Clinical Insight will guide you to the undestanding of the standard deviation and this NEW method of comparison MEASURED vs PREDICTED in Spirometry: the Z-Score and the LLN.

To define the LLN and the Z-Score is necessary to introduce the concept of standard deviation: “The STANDARD DEVIATION (SD) is a measure of how spread out numbers are from their EXPECTED MEAN VALUE”.

For example in case of dogs, using the STANDARD DEVIATION we have a “standard” way of knowing what is normal (Schnauzer), and what is extra large (Rottweiler) or extra small (Dachshund).

Fig. 1 – The heights of dogs at the shoulders in millimeters.

Normal Distribution and Percentile

When the data tend to be around a central value we have a NORMAL DISTRIBUTION with a bell–shaped curve.
Relationship between STANDARD DEVIATION (SD) and percentage of data under the curve in the case of a normal distribution.

When the data tend to be around a central value we have a NORMAL DISTRIBUTION with a bell–shaped curve.

68% of measures are between +1 and –1

90% of measures are between +1.64 and –1.64

Location and meaning of the 5th percentile
The range between the 5th percentile and 95th percentile includes 90% of the population.
The 5th percentile (-1.64 STANDARD DEVIATION) is the minimum value of the observed parameter below which is only 5% of the population.

Fig. 2 – Normal Distribution and Percentile.

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