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Ethnicity and z-score in spirometry

Normal distribution of residualsFig. 1 - Distribution of z-scores for FEV1 in healthy white females.

It does not do any harm to illustrate the usefulness of the z-score in lung function tests from yet another perspective. Going from left to right in figure 1, the z-scores relate to an ever increasing proportion of the population. Replace the absolute count with the cumulative percentage of the population on the Y-axis and you get the figure below. The scale is from 0 (0 subjects) to 1 (all subjects covered, 100% of the population). The cumulative frequency distribution in healthy white females (figure 2) is indistinguishable from that in black females. This illustrates once more the great utility of z-scores, as they can be interpreted independent of ethnic group.

Standardised residuals in African-American and swhite subjects







Fig. 2 - Cumulative frequency distribution of z-scores for FEV1 in healthy non-smoking white and black females.

 

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