Investigación clínica XVII Prueba Χ², de lo esperado a lo observado.

When you want to show if there is a statistical association or differences between categorical variables, it is recommended to use the Χ² test. This nonparametric test is one of the most used in clinical research; it contrasts nominal or ordinal qualitative variables that are observed in clinical practice. This test calculates the p value that determines whether differences between groups are real or due to chance. The Χ² test is the basis other tests to analyze qualitative ordinal variables as Χ² for linear trend, which compares three groups with two outcomes or McNemar test which contrasts two related samples (a before and afterward comparison) or Mantel-Haenszel Χ², which controls for potential confounding variables. When using small samples, where the expected is less than 5, Fisher's exact test should be used. These tests are the most widely used in the medical literature, however, do not give us the magnitude or the direction of the event and a proper interpretation that requires clinical judgment is needed. [ABSTRACT FROM AUTHOR]