title: A simple powerful bivariate test for two sample location problems in experimental and observational studies.
abstract: BACKGROUND
  In many areas of medical research, a bivariate analysis is desirable because it simultaneously tests two response variables that are of equal interest and importance in two populations. Several parametric and nonparametric bivariate procedures are available for the location problem but each of them requires a series of stringent assumptions such as specific distribution, affine-invariance or elliptical symmetry. The aim of this study is to propose a powerful test statistic that requires none of the aforementioned assumptions. We have reduced the bivariate problem to the univariate problem of sum or subtraction of measurements. A simple bivariate test for the difference in location between two populations is proposed.
  
  
  METHODS
  In this study the proposed test is compared with Hotelling's T(2) test, two sample Rank test, Cramer test for multivariate two sample problem and Mathur's test using Monte Carlo simulation techniques. The power study shows that the proposed test performs better than any of its competitors for most of the populations considered and is equivalent to the Rank test in specific distributions.
  
  
  CONCLUSIONS
  Using simulation studies, we show that the proposed test will perform much better under different conditions of underlying population distribution such as normality or non-normality, skewed or symmetric, medium tailed or heavy tailed. The test is therefore recommended for practical applications because it is more powerful than any of the alternatives compared in this paper for almost all the shifts in location and in any direction.
authors.affiliation: Department of Biostatistics, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Hamed
authors.initials: H
authors.lastname: Tabesh
authors.forename: S M T
authors.initials: SM
authors.lastname: Ayatollahi
authors.forename: Mina
authors.initials: M
authors.lastname: Towhidi
mesh_terms: D003627:Data Interpretation, Statistical; D008433:Mathematics
references.citation: Arch Environ Health. 1975 May;30(5):222-9
references.pmid: 1130834
references.citation: Am J Hum Biol. 2005 May-Jun;17(3):302-9
references.pmid: 15849710