Tests for Treatment Group Equality When Data are Nonnormal and Heteroscedastic

Cribbie, Robert A.; Wilcox, Rand R.; Bewell, Carmen; Keselman, H. J.

doi:10.22237/jmasm/1177992660

Cited by 30 publications

(24 citation statements)

References 35 publications

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“…The age data were nonnormal and heteroscedastic; therefore, the Welch test was applied to the ranked age values (as was recommended by Cribbie, Wilcox, Bewell, & Keselman, 2007). An omnibus test demonstrated that age differed significantly between the groups, F(2, 382.55) = 350.91, p < .001.…”

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confidence: 99%

An examination of the equivalency of self-report measures obtained from crowdsourced versus undergraduate student samples

Briones

Benham

2016

Behav Res

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Increasingly, researchers have begun to explore the potential of the Internet to reach beyond the traditional undergraduate sample. In the present study, we sought to compare the data obtained from a conventional undergraduate collegestudent sample to data collected via two online survey recruitment platforms. In order to examine whether the data sampled from the three populations were equivalent, we conducted a test of equivalency using inferential confidence intervals-an approach that differs from the more traditional null hypothesis significance testing. The results showed that the data obtained via the two online recruitment platforms, the Amazon Mechanical Turk crowdsourcing site and the virtual environment of Second Life, were statistically equivalent to the data obtained from the college sample, on the basis of means of standardized measures of psychological stress and sleep quality. Additionally, correlations between the sleep and stress measures were not statistically different between the groups. These results, along with practical considerations for the use of these recruitment platforms, are discussed, and recommendations for other researchers who may be considering the use of these platforms are provided.

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confidence: 99%

An examination of the equivalency of self-report measures obtained from crowdsourced versus undergraduate student samples

Briones

Benham

2016

Behav Res

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“…Because some species were more common than others, sample sizes were unequal and criteria for normality and heteroscedascity were not met. We therefore utilised the Welch test on ranked data to test for overall significance and the Ryan-Elinot-Gabriel-Welsch (REGWQ) test for pairwise comparisons; both of these tests have been shown to control Type I error rates and maximise power when distributions are skewed and variances are unequal (Cribbie et al, 2007).…”

Section: Statistical Analysesmentioning

confidence: 99%

Multi‐trophic impacts of an invasive aquatic plant

2016

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Summary Natural resource agencies are increasingly required to prioritise management of multiple non‐native aquatic plants (macrophytes) in freshwater ecosystems. Characterising the consequences of invasions for ecological processes and corresponding ecosystem services is fundamental to this decision‐making process, but requires an understanding of impacts across physical, chemical, and biological responses. Accordingly, we evaluated the multi‐trophic impacts of the invasive macrophyte Myriophyllum aquaticum (Haloragaceae; parrotfeather) along the mainstem Chehalis River, Washington, U.S.A. We examined invertebrate, plant and fish community responses to varying degrees of parrotfeather abundance and simultaneously characterised variation in physical structure and dissolved oxygen (DO) across the dominant native and non‐native plant species. DO concentrations were significantly reduced and approached hypoxic levels in areas dominated by parrotfeather compared with native vegetation. Increased structural complexity, volume and biomass of parrotfeather was associated with increased diversity of epiphytic invertebrates. However, these more diverse invertebrate assemblages were dominated by amphipods, whereas native macrophytes were characterised by cladocerans, chironomids and gastropods. Non‐native fishes (primarily centrarchid species) were more strongly associated with sites where parrotfeather was present and diversity of non‐native fishes was positively correlated along a gradient of parrotfeather abundance. Native fish associated with parrotfeather areas were those that tend to be tolerant of degraded or lower quality habitats. We saw little evidence of exclusion of other macrophytes; native and non‐native plant diversity and abundance were positively correlated with the parrotfeather gradient. This may reflect that analysis was done at a site level, and competitive dominance might be apparent by changes in species richness at smaller (plot) scales or over longer periods. Alternatively, parrotfeather may demonstrate minimal effects on native plant composition. Given the effects observed across multiple habitat characteristics and biota, parrotfeather appears to be a highly impactful invader where it establishes. Many of the changes we observed appear to derive from the emergent leaves and dense mat formation of parrotfeather compared with a submersed structure more typical of the native community. Our results suggest that managers should specifically consider contrasting characteristics between non‐native and native physical structure when assessing and prioritising threats of invasive macrophytes.

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“…In the literature, there are many studies in which the comparison of ANOVA and its alternative tests was made with respect to type I error rate and power when the assumptions of ANOVA are not satisfied. The effects of sample size (balanced/unbalanced case), non-normality, unequal variances, and combined effects of non-normality and unequal variances were investigated in details (Wilcox, 1988;Cribbie et al, 2012;Lantz, 2013;Gamage and Weerahandi, 1998;Parra-Frutos, 2013;Cribbie et al, 2007). In the light of these studies, we conduct a Monte Carlo simulation study in an attempt to give recommendations for applied researchers on the selection of appropriate one-way tests.…”

Section: Introductionmentioning

confidence: 99%

onewaytests: An R Package for One-Way Tests in Independent Groups Designs

Dağ¹,

Dolgun²,

Konar³

2018

The R Journal

124

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One-way tests in independent groups designs are the most commonly utilized statistical methods with applications on the experiments in medical sciences, pharmaceutical research, agriculture, biology, engineering, social sciences and so on. In this paper, we present the onewaytests package to investigate treatment effects on the dependent variable. The package offers the one-way tests in independent groups designs, which include ANOVA, Welch's heteroscedastic F test, Welch's heteroscedastic F test with trimmed means and Winsorized variances, Brown-Forsythe test, Alexander-Govern test, James second order test and Kruskal-Wallis test. The package also provides pairwise comparisons, graphical approaches, and assesses variance homogeneity and normality of data in each group via tests and plots. A simulation study is also conducted to give recommendations for applied researchers on the selection of appropriate one-way tests under assumption violations. Furthermore, especially for non-R users, a user-friendly web application of the package is provided. This application is available at

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Tests for Treatment Group Equality When Data are Nonnormal and Heteroscedastic

Cited by 30 publications

References 35 publications

An examination of the equivalency of self-report measures obtained from crowdsourced versus undergraduate student samples

An examination of the equivalency of self-report measures obtained from crowdsourced versus undergraduate student samples

Multi‐trophic impacts of an invasive aquatic plant

onewaytests: An R Package for One-Way Tests in Independent Groups Designs

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