Article

Nakagawa S, Cuthill IC.. Effect size, confidence intervals and statistical significance: a practical guide for biologists. Biol Rev Camb Philos Soc 82: 591-605

Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
Biological Reviews (Impact Factor: 9.67). 12/2007; 82(4):591-605. DOI: 10.1111/j.1469-185X.2007.00027.x
Source: PubMed

ABSTRACT

Null hypothesis significance testing (NHST) is the dominant statistical approach in biology, although it has many, frequently unappreciated, problems. Most importantly, NHST does not provide us with two crucial pieces of information: (1) the magnitude of an effect of interest, and (2) the precision of the estimate of the magnitude of that effect. All biologists should be ultimately interested in biological importance, which may be assessed using the magnitude of an effect, but not its statistical significance. Therefore, we advocate presentation of measures of the magnitude of effects (i.e. effect size statistics) and their confidence intervals (CIs) in all biological journals. Combined use of an effect size and its CIs enables one to assess the relationships within data more effectively than the use of p values, regardless of statistical significance. In addition, routine presentation of effect sizes will encourage researchers to view their results in the context of previous research and facilitate the incorporation of results into future meta-analysis, which has been increasingly used as the standard method of quantitative review in biology. In this article, we extensively discuss two dimensionless (and thus standardised) classes of effect size statistics: d statistics (standardised mean difference) and r statistics (correlation coefficient), because these can be calculated from almost all study designs and also because their calculations are essential for meta-analysis. However, our focus on these standardised effect size statistics does not mean unstandardised effect size statistics (e.g. mean difference and regression coefficient) are less important. We provide potential solutions for four main technical problems researchers may encounter when calculating effect size and CIs: (1) when covariates exist, (2) when bias in estimating effect size is possible, (3) when data have non-normal error structure and/or variances, and (4) when data are non-independent. Although interpretations of effect sizes are often difficult, we provide some pointers to help researchers. This paper serves both as a beginner's instruction manual and a stimulus for changing statistical practice for the better in the biological sciences.

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    • "An effect size estimator (assorted with 95% confidence interval, thereafter noted 95%CI) was further used to compare the 3 averaged traits between V. crabro and V. velutina (Nakagawa & Cuthill, 2007). Cliff's delta estimator was preferred to usual Cohen's d because our data were nonparametric (Cliff, 1996; Nakagawa & Cuthill, 2007; Macbeth et al., 2010; Ivarsson et al., 2013). "
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    ABSTRACT: Recent studies have focused on the role of behavior in biological invasions. Individuals may differ consistently in time for several behavioral traits (personality) which covary (behavioral syndrome) resulting in different behavioral types, some of them favoring invasion. Social hymenopterans have a strong potential to be invaders and their success depends primarily on the foundresses’ ability to found viable colonies. They are expected to be active, explorative and bold for optimally establishing their nest. In Europe, 2 hornet species coexist: the native Vespa crabro and the invasive Vespa velutina. These 2 species may compete for nesting sites and we suggest that the initial success of V. velutina has been favored by its behavior in outperforming V. crabro for the traits involved in nest initiation. Here, we (i) defined the personality of V. crabro and V. velutina, (ii) tested for the existence of behavioral syndrome in these species, and (iii) compared their performances using an open-field test. Our results show that V. crabro foundresses behave consistently but not V. velutina; this lack of consistency being mainly due to reduced variance among individuals. This result questions the possibility of detecting consistent behavioral differences in species having recently undergone a strong bottleneck. Both species exhibit the same correlations between activity, boldness and exploration and V. velutina clearly outperforms V. crabro for all traits. Our results suggest that activity, boldness, and exploration are implicated in both hornet nest initiation and invasion process which contributed to explain why social hymenopterans are so successful at colonization. Key words: animal personality; biological invasion; invasion syndrome; Vespa crabro; Vespa velutina; Vespidae
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    • "During the breeding season, from April to August, we checked every nestbox and other potential nest sites for active nests. We identified social parents by behavioural observations of each nest (Nakagawa et al. 2007). We collected tissue samples for DNA extraction from adults, chicks and unhatched eggs. "

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    • "An effect size estimator (assorted with 95% confidence interval, thereafter noted 95%CI) was further used to compare the 3 averaged traits between V. crabro and V. velutina (Nakagawa & Cuthill, 2007). Cliff's delta estimator was preferred to usual Cohen's d because our data were nonparametric (Cliff, 1996; Nakagawa & Cuthill, 2007; Macbeth et al., 2010; Ivarsson et al., 2013). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent studies have focused on the role of behavior in biological invasions. Individuals may differ consistently in time for several behavioral traits (personality) which co-vary (behavioral syndrome) resulting in different behavioral types, some of them favoring invasion. Social hymenopterans have a strong potential to be invaders and their success depends primarily on the foundresses' ability to found viable colonies. They are expected to be active, explorative and bold for optimally establishing their nest. In Europe, two hornet species coexist: the native Vespa crabro and the invasive V. velutina. These two species may compete for nesting sites and we suggest that the initial success of V. velutina has been favored by its behavior in outperforming V. crabro for the traits involved in nest initiation. Here, we (i) defined the personality of V. crabro and V. velutina, (ii) tested for the existence of behavioral syndrome in these species and, (iii) compared their performances using an open-field test. Our results show that V. crabro foundresses behave consistently but not V. velutina; this lack of consistency being mainly due to reduced variance among individuals. This result questions the possibility of detecting consistent behavioral differences in species having recently undergone a strong bottleneck. Both species exhibit the same correlations between activity, boldness and exploration and V. velutina clearly outperforms V. crabro for all traits. Our results suggest that activity, boldness and exploration are implicated in both hornet nest initiation and invasion process which contributed to explain why social hymenopterans are so successful at colonization. This article is protected by copyright. All rights reserved.
    Full-text · Article · Jul 2015 · Insect Science
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