The averages and standard deviations of several subgroups in a study are sometimes need to be combined in meta-analysis, the calculating method to do this was given in this paper. Firstly, a commonly used incorrect calculating method to do the combination was pointed out; it origins from metrology and should be applied in the combination of standard deviations of several groups of measurements of the same object, and it's not fit for the combination of several subgroups' data in meta-analysis. Meanwhile the calculating error of this method was tested with real and constructed data and its impact on the conclusion of meta-analysis was analyzed. The incorrect method may result in a big calculating error with the real research data and even lead to an incorrect conclusion with the constructed data. The correct formula was deduced and given, the calculating result of the correct formula equals the true value when neglecting the round error. The error analysis of the incorrect method was then done with the correct formula and an accurate combination result can only be got when meeting the condition given by this paper. The correct method in this paper should be used to combine standard deviations in the future researches.
Objective To explore the diagnostic value on 24 hour monitoring of intraocular pressure (IOP) for primary open angle glaucoma (POAG). Methods A prospective study was applied, and a total of 372 subjects through January 2012 to May 2015 for 24 hour IOP monitoring were collected successively, including 137 subjects (271 eyes) with glaucoma (glaucoma group) and 235 subjects (470 eyes) with non-glaucoma (Control group). Data was analyzed using SPSS 13.0 software, and the Kappa statistics was used to evaluate concordance between 24 hour monitoring of IOP and gold standard for POAG diagnosis. Results The mean value of IOP at all monitoring period in glaucoma group was significant higher than that in the control group (P < 0.001). The peak of IOP occurred at 6:00 am and 10:00 in the glaucoma group, and the fluctuation value of IOP in women patients at night (especially at 22:00 pm) was higher than that of men (t=2.064, P=0.04). The sensitivity and specificity of 24 hour IOP monitoring for POAG were 97% and 78.7%, respectively, and with a high consistency comparing to the result of gold standard for POAG diagnosis, with the Kappa values of 0.707 (P < 0.000 1). Conclusion 24 hour IOP monitoring is efficacy and convenient tool, which can be applied alone or combined with other tools to assist early diagnosis patients who are suspected with POAG, so as to improve the diagnostic accuracy.