The Performance of Multivariate Methods for Two-Group Comparisons with Small Samples and Incomplete Data

Abstract

In intervention studies having multiple outcomes, researchers often use a series of univariate tests (e.g., ANOVAs) to assess group mean differences. Previous research found that this approach properly controls Type I error and generally provides greater power compared to MANOVA, especially under realistic effect size and correlation combinations. However, when group differences are assessed for a specific outcome, these procedures are strictly univariate and do not consider the outcome correlations, which may be problematic with missing outcome data. Linear mixed or multivariate multilevel models (MVMMs), implemented with maximum likelihood estimation, present an alternative analysis option where outcome correlations are taken into account when specific group mean differences are estimated. In this study, we use simulation methods to compare the performance of separate independent samples t tests estimated with ordinary least squares and analogous t tests from MVMMs to assess two-group mean differences with multiple outcomes under small sample and missingness conditions. Study results indicated that a MVMM implemented with restricted maximum likelihood estimation combined with the Kenward–Roger correction had the best performance. Therefore, for intervention studies with small N and normally distributed multivariate outcomes, the Kenward–Roger procedure is recommended over traditional methods and conventional MVMM analyses, particularly with incomplete data.     

Limitations of a Bidirectional Control Procedure for the Investigation of Imitation in Rats: Odour Cues on the Manipulandum

Magazine-trained observer rats confronted a conspecific demonstrator pushing a joystick to the right or to the left for food reward before the observers were given access to the joystick from the position previously occupied by the demonstrator and rewarded for responses in both directions. For half of the observers (group 0), the joystick was in the same position when acted upon by demonstrators and observers; for the other half (group 180) the manipulandum was rotated 180 within its mounting between observation and test. As in previous experiments using this bidirectional control procedure, rats in group 0 showed demonstratorconsistent responding-that is, they pushed the joystick in the same direction, relative to the actor's body, as had their demonstrators. However, group 180 showed a reverse effect: reliable demonstrator-inconsistent responding. These results suggest that attractive odour or taste cues deposited by demonstrators on the side of the joystick contralateral to the direction of responding are sufficient to produce demonstrator-consistent responding in the bidirectional control procedure.