Accuracy of Effect Size Estimates From Published Psychological Experiments Involving Multiple Trials

The reporting of exaggerated effect size estimates may occur either through researchers accepting statistically significant results when power is inadequate and/or from repeated measures approaches aggregating, averaging multiple items, or multiple trials. Monte-Carlo simulations with input of a small, medium, or large effect size were conducted on multiple items or trials that were either averaged or aggregated to create a single dependent measure. Alpha was set at the .05 level, and the trials were assessed over item or trial correlations ranging from 0 to 1. Simulations showed a large increase in observed effect size averages and the power to accept these estimates as statistically significant increased over numbers of trials or items. Overestimation effects were mitigated as correlations between trials increased but still remained substantial in some cases. The implications of these findings for meta-analyses and different research scenarios are discussed.     

Correcting Overestimated Effect Size Estimates in Multiple Trials

In a simulation study, Brand, Bradley, Best, and Stoica (2011 Brand, A., Bradley, M. T., Best, L. A. and Stoica, G. 2011. Multiple trials may yield exaggerated effect size estimates. The Journal of General Psychology, 138: 1–11. [Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]) have shown that Cohen's d is notably overestimated if computed for data aggregated over multiple trials. Although the phenomenon is highly important for studies and meta-analyses of studies structurally similar to the simulated scenario, the authors do not comprehensively address how the problem could be handled. In this comment, we first suggest a corrective term d ^′ _c that includes the number and correlation of trials. Next, the results of a simulation study provide evidence that the proposed d_c′ results in a more precise estimation of trial-level effects. We conclude that, in practice, d ^′ _c together with plausible estimates of inter-trial correlation will produce a more precise effect size range compared to that suggested by Brand and colleagues (2011 Brand, A., Bradley, M. T., Best, L. A. and Stoica, G. 2011. Multiple trials may yield exaggerated effect size estimates. The Journal of General Psychology, 138: 1–11. [Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]).     

How Accurate Is the Pearson r-from-Z Approximation? A Monte Carlo Simulation Study

The Pearson r-from-Z approximation estimates the sample correlation (as an effect size measure) from the ratio of two quantities: the standard normal deviate equivalent (Z-score) corresponding to a one-tailed p-value divided by the square root of the total (pooled) sample size. The formula has utility in meta-analytic work when reports of research contain minimal statistical information. Although simple to implement, the accuracy of the Pearson r-from-Z approximation has not been empirically evaluated. To address this omission, we performed a series of Monte Carlo simulations. Results indicated that in some cases the formula did accurately estimate the sample correlation. However, when sample size was very small (N = 10) and effect sizes were small to small-moderate (ds of 0.1 and 0.3), the Pearson r-from-Z approximation was very inaccurate. Detailed figures that provide guidance as to when the Pearson r-from-Z formula will likely yield valid inferences are presented.     

Do you see what I see?: An exploration of inter-ethnic ideal body size comparisons among college women

The present study explored African American (n = 16) and European American (n = 19) college women's ideal body size perceptions for their own and the other ethnic group along with reasons behind their selections. Respondents completed an ethnically-neutral figure rating scale and then participated in ethnically-homogenous focus groups. European Americans mostly preferred a curvy-thin or athletic ideal body while most African American students resisted notions of a singular ideal body. European Americans suggested that African Americans’ larger ideal body sizes were based on greater body acceptance and the preferences of African American men. African Americans used extreme terms when discussing their perceptions of European Americans’ thin idealization, celebrity role models, and weight management behaviors. African Americans’ perceptions of European Americans’ body dissatisfaction were also attributed to the frequent fat talk they engaged in. Implications for promoting the psychosocial well-being of ethnically-diverse emerging adult females attending college are discussed.     

Interpreting effect sizes when controlling for stability effects in longitudinal autoregressive models: Implications for psychological science

Effect sizes in longitudinal studies often are dramatically smaller than effect sizes in cross-sectional studies. Indeed, autoregressive models (which are often used in longitudinal studies but not in cross-sectional studies) control for past levels on the outcome (i.e., stability effects) in order to predict change in levels of the outcome over time and thus may greatly reduce the magnitude of the effect of a predictor on the outcome. Unfortunately, however, there have been no attempts to differentiate guidelines for interpreting effect sizes for longitudinal studies versus cross-sectional studies. Consequently, longitudinal effect sizes that fall below the universal guidelines for “small” may be incorrectly dismissed as trivial, when they might be meaningful. In the current paper, we first review the present guidelines for interpreting effect sizes. Next, we discuss several examples of how controlling for stability effects can dramatically attenuate effect sizes of other predictors, in order to support our argument that the current guidelines may be misleading for interpreting longitudinal effects. Finally, we conclude by making recommendations for researchers regarding the interpretation of effect sizes in longitudinal autoregressive models.     

Study of incommensurate modulation in V-doped La2Cu1- x (VO) x O4 + δ by transmission electron microscopy

Structural variations in a hole-rich grain of V-doped La₂Cu_{1? x} (VO) _x O_{4+ δ} (x?=?0.045, δ?=?0.011) superconducting compound have been studied by electron diffraction and high-resolution transmission electron microscopy. Two types of one-dimensional (1-D) incommensurate modulation were observed one after the other when the specimen was exposed to electron beam irradiation. The features of these two modulated structures have been studied and are discussed.     

Positron lifetime study of an Al-1.7at.% Mg-1.1at.% Cu alloy

The behaviour of vacancies during isothermal ageing following quenching of an Al-1.7at.% Mg-1.1at.% Cu alloy has been studied by positron annihilation lifetime spectroscopy. The positron lifetime parameters vary in parallel with the Vickers hardness of the alloy, suggesting simultaneous migration of vacancies and changes in the size and concentration of vacancies containing clusters and dislocation loops. The results also explain the long hardness plateau observed previously in ageing experiments carried out between 100 and 240°C in terms of a continuous growth of Mg-Cu-vacancy clusters during ageing. The vacancy concentration of the clusters increases gradually until the setting in of the cluster Guinier-Preston-Bagaryatsky zone transformation.     

Nanoindentation study of size effects in nickel–graphene nanocomposites

Metal–graphene nanocomposites find applications in nanoscale devices, as functional materials and can serve as a test bed to gain insight into fundamental deformation mechanisms of metals under geometric confinement. Here, we report full atomistic nanoindentation simulations for nickel–graphene nanocomposites with varied numbers of layers of graphene sheets to investigate the size effects on the hardness, and to understand how emerging dislocation loops interact with the nickel–graphene interface under varied geometric confinements. A detailed analysis of the plastic deformation mechanism shows that as dislocation loops reach the nickel–graphene interface, the local bending of the graphene sheet is altered and further dislocation propagation is blocked. An increase in the number of graphene layers decreases the hardness, but increases the maximum elastic deformation of the nickel–graphene nanocomposites. These findings indicate that the mechanical properties of nickel–graphene nanocomposites can be engineered by controlling the thickness of nickel and graphene layers, respectively.     

Influence of test temperature on the size effect in molybdenum small-scale compression pillars

Previous research has shown that body-centred cubic (bcc) metals exhibit a smaller size dependence of strength than what is commonly observed in face-centred cubic (fcc) metals. This work investigates compression testing of focused ion beam-manufactured molybdenum pillars ranging in size from 300?nm to 5?μm, both above and below its critical temperature at 300 and 500?K. At 500?K the size effect is found to be consistent with what is observed in fcc metals, owing to the increased mobility of screw dislocations.