Consider a situation in which we would like to evaluate the adequacy between hemoglobin measurements (in g/dl) with a hemoglobinometer on the hospital bed and the formal photometric laboratory technique in ten people [Table 3]. The Bland Altman diagram for this data shows the difference between the two methods for each person [Figure 1]. The mean difference between the values is 1.07 g/dl (with a standard deviation of 0.36 g/dL) and the 95% match limits are 0.35 to 1.79. This means that the hemoglobin level measured by a given person`s photometry can vary from 0.35 g/dl greater than 1.79 g/dl measured by photometry (this is the case for 95% of people; for 5% of individuals, variations could be outside these limits). This obviously means that the two techniques cannot be used as substitutes. It is important that there is no single criterion for acceptable compliance limits; This is a clinical decision that depends on the variables to be measured. There are two groups of variables that you need to know: categorical variables and continuous variables. We use the word variable groups because kategorial and continuous variables contain additional types of variables. However, there may be some ambiguity as to whether a variable is categorical or continuous. We discuss the two groups of variables as well as these potential ambiguities in the following sections: you can learn more about the different uses of variables, especially in quantitative research designs (i.e.
descriptive, experimental, quasi-experimental and relational research designs), in the section on research designs. It is important to note that in each of the three situations in Table 1, the percentages of success are the same for both examiners, and if both examiners are compared to a usual 2 × 2 test for matched data (McNemar test), there would be no difference in their performance; on the other hand, agreement among observers varies considerably from country to country in all three situations. The fundamental concept is that “convergence” quantifies the concordance between the two examiners for each of the “pairs” of marks and not the similarity of the overall percentage of success between the examiners. . . .