parameter(z)

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About This Site

I created this site because I wanted to better understand just what the heck it was that we were calling "normal":

the definition of abnormal relies on the definition of normal1

After all, this distinction is kind of why we are making these measurements.

Enter z-scores: "Just get the z-score, and you'll know if it's normal or not."

Simple enough, right?

About Z-Scores

Z-scores (or standard scores) describe the distance, in standard deviations, a given measurement is from the mean value. The WikiPedia entry provides insight into something foundational about the concept:

the use of "Z" is because the normal distribution is also known as the "Z distribution"

We are fond of using z-scores in pediatric cardiology because, theoretically, they help us deal with the confounding effect of normal somatic growth. However, not all z-scores equations are created equal. A recent editorial2 and critical reviews3 expose these inequalities and help set the current state of the art in this field.
Interested persons should read them.

About Me

My name is Dan Dyar. I became interested in studying hearts at about the time echocardiography was just hitting its stride. I have always wondered about how we, as a community and industry, could have developed such a sophisticated tool yet often we have a primitive (if not frankly incorrect) approach to classifying normal and abnormal findings.

I like to think that by providing these calculators on the web, not only will I have personally learned more about reference values, but maybe others will be inspired to deepen their own understanding about the very point of measurement in echocardiography.

View my CV here.

  1. Normal reference ranges for echocardiography: do we really need more? ↩

  2. I found the recent "why and how of Z scores" editorial a little bit confusing—by mistakenly substituting BMI (instead of BSA) as the size-adjusting variable— but overall an excellent discussion (sadly, even the later correction is still in need of correction). ↩

  3. Dallaire and Dahdah basically schooled us all on what to (and not to) do in their coronary artery manifesto manuscript; that same group's harsh but fair criticism of statistical methods and the recent critique of measurement methodologies by Cantinotti et al. should, in my opinion, be required reading for students of pediatric echocardiography. ↩

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