Validating the bland altman method of agreement

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Quote: Angiocardiograms are radiographs made while a radiopaque medium is circulating through the heart (and closely associated vessels). Breed-specific prevalence rates were significantly greater in purebred dogs than in mongrels, and the breed distributions of patent ductus arteriosus, pulmonic stenosis, subaortic stenosis, persistent right aortic arch, and tetralogy of Fallot were significantly different than would be expected if all breeds were equally susceptible to each type of malformation. Patent ductus arteriosus, as in man, was found predominantly in females. The principles in selecting the chamber or vessel for the injection are based primarily upon the knowledge that cardiac valves are normally unidirectional and blood pressures in the left atrium, left ventricle, and aorta normally exceed the pressures in corresponding right heart structures. In dogs with suspected valvular insufficiencies, injections are made into the vessel or cardiac chamber from which the contrast medium would regurgitate (i.e., immediately “down stream” from the affected valve). (2) These genetic factors have specific effects on cardiac morphogenesis, resulting in specific types of cardiovascular malformations. L Watkins, Jr, J A Burton, E Haber, J R Cant, F W Smith, A C Barger. Constriction of the pulmonary artery or thoracic inferior vena cava was maintained for 2 wk while daily measurements were made of plasma renin activity, plasma aldosterone, plasma volume, hematocrit, serum sodium and potassium concentrations, sodium and water balance, body weight, and arterial, caval, and atrial pressures. In selective angiocardiography, the contrast medium is injected through a catheter, the tip of which has been positioned into a specific vessel or cardiac chamber to visualize best a suspected abnormality based upon the clinical findings. On the basis of these observations, two hypotheses were made: (1) Genetic factors are determinants of certain types of congenital heart disease in the dog. Quote: "It was considered that the relationship between dp/dt and simultaneously developed pressure during the course of isovolumic contraction might afford a more accurate measure of contractility than the maximum rate of intraventricular pressure rise (peak dp/dt). In conclusion, the determination of dp/dt and intraventricular pressure throughout isovolumic contraction in the presence of variable arterial pressure and small changes of preload provides a useful, simple, and experimentally verified approach to the assessment of alterations of the contractile state of the heart in intact man." . Quote: "The role of the renin-angiotensin-aldosterone system in the development of congestive failure has been assessed in the conscious dog by use of the nonapeptide converting enzyme inhibitor.It is identical to a Tukey mean-difference plot Bland and Altman make the point that any two methods that are designed to measure the same parameter (or property) should have good correlation when a set of samples are chosen such that the property to be determined varies considerably.A high correlation for any two methods designed to measure the same property could thus in itself just be a sign that one has chosen a widespread sample.Bland-Altman plots allow identification of any systematic difference between the measurements (i.e., fixed bias) or possible outliers.

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The Bland-Altman plot pairs observations across methods and plots their mean values vs. This method provides only limited information, however, when the range of observed values is small relative to the number of observations.

It is common to compute 95% limits of agreement for each comparison (average difference ± 1.96 standard deviation of the difference), which tells us how far apart measurements by 2 methods were more likely to be for most individuals.

If the differences within mean ± 1.96 SD are not clinically important, the two methods may be used interchangeably.

The 95% limits of agreement can be unreliable estimates of the population parameters especially for small sample sizes so, when comparing methods or assessing repeatability, it is important to calculate confidence intervals for 95% limits of agreement.

This can be done by Bland and Altman's approximate method Bland-Altman plots were also used to investigate any possible relationship of the discrepancies between the measurements and the true value (i.e., proportional bias).

This brief report shows how adding a simple bar chart to a BlandAltman plot adds essential additional information.

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