During the work, certain metabolites in blood plasma are studied (more specifically, the concentrations of metabolites are measured). A test sample and a control group were collected. One of the tasks: To analyze the correlations between the age and significant clinical data groups of the subjects with BRCA1 / 2 positive breast tumor and the changes in amino acids (profile) and in comparison with the control group. (Relevant clinical data here include: cancer stage and treatment stage). For the calculation, I also analyzed the differences in the study sample itself, i.e., whether there was a statistically significant difference, e.g., between the plasma metabolite concentrations of stage I and II patients. The key question I received from my supervisor is: Is it correct to compare the data obtained (metabolite concentrations) in the study group (between subgroups such as different stages and treatment stage) if I compare them later with the control group? maybe someone can explain whether it is correct to compare subgroups within the study sample itself?