Reliability is defined as the degree to which a measurement technique can achieve consistent results when repeated measurements on the same objects, either by several evaluators or by an observer at different times. It is necessary to distinguish two different types of reliability; Consistency or absolute consistency. For example, three evaluators independently evaluate twenty students` applications for a scholarship on a scale of zero to 100. The first evaluator is particularly tough and the third is particularly lenient, but each evaluator scores consistent points. There must be differences between the actual scores given by the three evaluators. If the goal is to evaluate candidates and select five students, the difference between evaluators cannot lead significantly to different results if „consistency“ has been maintained throughout the evaluation process. However, if the goal is to select students assessed above or below a preset default absolute score, the results of the three assessors at the mathematical level must be absolutely similar. Therefore, while we want a uniform assessment in the first case, we want to reach an `absolute agreement` at a later stage. The different objective is reflected in the procedure for calculating reliability. which can be interpreted as the fraction of the total variance due to the variation between groups. In his classic book, Ronald Fisher devotes an entire chapter to intraclassical correlation.  In summary, CCI is a reliability index that reflects both the degree of correlation and the concordance between measures. It has been used frequently in conservative care medicine to assess the reliability of numerical or continuous measurements.
Since there are 10 different forms of CCI and each form has different assumptions in their calculations and gives rise to different interpretations, it is important for researchers and readers to understand the principles of selecting an appropriate ICC form. Since the icc estimate obtained from an assurance study is only an estimated value of the actual CCI, it is more appropriate to assess the degree of reliability on the basis of the 95% confidence interval of the ICC estimate and not the ICC estimate itself. Reliability, based on absolute consistency, is always lower than consistency, since a stricter criterion is applied. A clinical researcher has developed a new ultrasound-based method to quantify coliotic deconformity. Before using the new method for his routine clinical practice, he conducted a reliability study to assess the reliability of the tests and repeat tests. He recruited 35 scoliosis patients with a variety of deformities in a pediatric hospital and used his new method to measure their coliotic deformity. Measurements were repeated 3 times for each patient. He analyzed his data with a model of unique and absolute agreements, 2 ways-mixed effects and reported his ICC results in a peer review journal as ICC = 0.78 with 95% confided interval = 0.72-0.84. .