Graphical Display and Statistical Evaluation of Data Gained by Long-Term Ambulatory Intragastric pH Monitoring

Abstract

The graphical display of data gained by long-term ambulatory intragastric pH monitoring requires different methods to present (1) the circadian time course of each individual, (2) data from all individuals belonging to the same group, and (3) data from two or more groups in a summary plot. Data reduction is normally required in all three cases; for this purpose, it is advisable to use median values from consecutive time periods, e.g. of 10-20 min each. All data from a total group of comparable individuals may be displayed using a pseudo-3D-graph allowing for easy recognition of interindividual variability. Summary curves from all data of one group may be gained by calculating medians from time-equivalent pH values and plotted together in one graph. The study design should take into consideration that intraindividual reproducibility of intragastric pH metry is excellent, while large variations may occur between individuals. Therefore, repeated measurements in the same individual under the different conditions to be tested are highly recommended. If two measurements are performed in each individual, the data are statistically evaluated using Wilcoxon paired rank test; Friedman rank test and subsequent multiple comparison according to Wilcoxon-Wilcox are proposed in the case of more than two groups. Alternatively, multiple single tests may be used; this procedure, however, requires appropriate correction of the alpha-error which is best done using the exact versions of the test.