A Note on the Concept of Normality and Abnormality in Quantitation of Pathologic Findings in Congenital Heart Disease

A NOTE ON THE CONCEPT OF NORMALITY AND ABNORMALITY IN qUANTITATION OF PATHOLOGIC FINDINGS IN CONGENITAL HEART DISEASE HORACIO

J.

A. RIMOLDI, M.D.

MAURICE LEV, M.D.

In previous studies 1 • 2 a method was evolved for qualitative and quantitative study of the pathologic change in congenital heart disease. This involved measurements of heart weight, weight of parietal walls, size of chambers, thickness of walls, and sizes of orifices. The point of reference was the study of the measurements of these modalities in 83 hearts of normal children from birth to 15 years of age. Equations were derived making possible the prediction of the mean value and the 5 per cent level of confidence for any modality studied in a child of certain height, weight and age. The present communication deals with the question as to how to reach the final judgment of normality or abnormality from these predicted "normal" values for the child in question. From the Congenital Heart Disease Research and Training Center, Hektoen Institute for Medical Research, and the Departments of Pathology of Northwestern University Medical School, University of Chicago School of Medicine and the Loyola Psychometric Laboratory of Loyola University, Chicago. This investigation was supported by a Research Grant (HE-07605-1, formerly H-3351) from the National Heart Institute of the National Institutes of Health, United States Public Health Service, Bethesda, Md.

590

QUANTITATIVE PATHOLOGY OF CONGENITAL HEART DISEASE

EVALUATION

The assumption is made that the variables in question follow closely a normal distribution. This assumption was not verified experimentally, but we believe that it may be defended on logical grounds. In order to establish categories that would be useful for the purposes of judgments in pathology, the following nomenclature was decided upon: normal, probably normal, probably abnormal and abnormal. l. We are considering normal any value between the mean of the distribution and +0.6745u and -0.6745u. As indicated in Figure 1, this area includes 50 per cent of the cases within the normal range. These measures were arbitrarily chosen because they have been widely used in statistical theory where they are known as probable error and third and first quartiles for +0.6745u and -0.6745u respectively. In spite of the fact that the concept of probable error tends to disappear from modern statistical theory, it is still a valid value to indicate the 50 per cent of the cases that center around the mean of a normal distribution. 95% I

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01 (b"Jartiie) Q3(g~~rliie) Fig. 1. Criteria for the diagnosis of normal, abnormal, probably normal and probably abnormal.

2. We are considering abnormal any value at or beyond +1.96u from the mean. This corresponds to the commonly used 5 per cent level of significance, of which 2.5 per cent will be at the higher and 2.5 per cent at the lower end of the distribution. The area between +I.96u and -l.96u includes 95 per cent of the cases. For convenience one might use values of +2.00u and -2.00u, which would produce an error of approximately 0.2 per cent. 3. The previous limits between the normal and the abnormal leave an area that includes 22.5 per cent on each half of the distribution. We are arbitrarily dividing the areas between 0.6745u and 1.96u and between -0.6745u and -1.96u into equal parts. This corresponds to the values

HORACIO

J.

A. RIMOLDI AND MAURICE LEV

591

+l.IOa and -l.lOa for the right and left tails of the distribution, leaving approximately 11.25 per cent of the cases in each new category. We are considering probably normal any value between 0.6745a and l.lOOa or between -0.6745a and -l.IOOa. We are considering probably abnormal any value between l.lOa and l.96a or between -l.lOa and -1.96a. CONCLUSION

The meaning of judgments in the quantitative pathology of congenital heart disease are elucidated. The terms "normal," "probably normal," "abnormal," and "probably abnormal" are defined. REFERENCES

1. Lev, M .. Rowlatt, U. F., and Rimoldi, H. J. A.: Pathologic Methods for the Study of the Congenitally Malformed Heart: Methods for Electrocardiographic and Physiologic Correlation. A. M.A. Arch. Path., 72:493, 1961. 2. Rowlatt, U. F., Rimoldi, H. J. A., and Lev, M.: The Quantitative Anatomy of the Normal Child's Heart. PEDIAT. CLIN. N. AMER., 10:499, 1963 (this issue). Loyola Psychometric Laboratory (Dr. Rimoldi) Chicago, Ill. 629~637 S. Wood St. (Dr. Lev) Chicago 12, Ill.