Mixed-model pairwise multiple comparisons of repeated measures means.

One approach to the analysis of repeated measures data allows researchers to model the covariance structure of the data rather than presume a certain structure, as is the case with conventional univariate and multivariate test statistics. This mixed-model approach was evaluated for testing all possible pairwise differences among repeated measures marginal means in a Between-Subjects?×?Within-Subjects design. Specifically, the authors investigated Type I error and power rates for a number of simultaneous and stepwise multiple comparison procedures using SAS (1999) PROC MIXED in unbalanced designs when normality and covariance homogeneity assumptions did not hold. J. P. Shaffer's (1986) sequentially rejective step-down and Y. Hochberg's (1988) sequentially acceptive step-up Bonferroni procedures, based on an unstructured covariance structure, had superior Type I error control and power to detect true pairwise differences across the investigated conditions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

On the power of multivariate latent growth curve models to detect correlated change.

We evaluated the statistical power of single-indicator latent growth curve models (LGCMs) to detect correlated change between two variables (covariance of slopes) as a function of sample size, number of longitudinal measurement occasions, and reliability (measurement error variance). Power approximations following the method of Satorra and Saris (1985) were used to evaluate the power to detect slope covariances. Even with large samples (N=500) and several longitudinal occasions (4 or 5), statistical power to detect covariance of slopes was moderate to low unless growth curve reliability at study onset was above .90. Studies using LGCMs may fail to detect slope correlations because of low power rather than a lack of relationship of change between variables. The present findings allow researchers to make more informed design decisions when planning a longitudinal study and aid in interpreting LGCM results regarding correlated interindividual differences in rates of development. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Randomization tests for predicted trends.

Argues that randomization tests can utilize a predicted trend to increase their power, much as a predicted direction of difference is incorporated into a t test to increase its power. The tests permit very specific predictions and consequently are quite powerful when the predictions have a firm basis. Randomization tests for linear and quadratic trends are described. Comparisons of probability values given by analysis of variance with those of a randomization test sensitive to quadratic U-shaped trends are provided for actual research data, to indicate the power of the randomization tests for trend. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Power of univariate pairwise multiple comparison procedures.

Used Monte Carlo simulation to evaluate the power of various procedures under the constraint of experimentwise Type I error control. Both the configuration of population means and the method of assessing power influenced the relative powers of the procedures. The Shaffer-Welsch or one of several Q-test procedures had the greatest any-pair power or power to detect at least true difference. Either Peritz's F-test procedure or a model testing procedure had the greatest all-pairs power or power to detect all true differences. (29 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Testing for substantive significance in applied research by specifying nonzero effect null hypotheses.

Contends that researchers using ANOVA, who wish to qualify the reporting of a statistically significant but nonsubstantive treatment effect, may test a null hypothesis that specifies the minimum effects size considered important and then reject or accept that hypothesis with the same confidence ensuing from a test of traditional null hypothesis of no difference in population means. A method is proposed that uses a simplified approximate solution, since appropriate noncentral F distributions are not generally available. (19 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Calibration of Water Distribution Hydraulic Models Using a Bayesian-Type Procedure

Estimating model parameters is a difficult, yet critical step in the use of water distribution system models. Most of the optimization-based approaches developed so far concentrate primarily on efficient and effective ways of obtaining optimal calibration parameter values. At the same time, very little effort has been made to determine the uncertainties (i.e., errors) associated with those values (and related model predictions). So far, this has typically been done using the first-order second moment (FOSM) method. Even though reasonably computationally efficient, the FOSM approach relies on several restrictive assumptions and requires computationally demanding calculation of derivatives. To overcome these limitations, the recently developed shuffled complex evolution metropolis (SCEM-UA) global optimization algorithm is linked to the Epanet2 hydraulic model and used to solve a least-squares-type calibration problem. The methodology is tested and verified on the Anytown literature case study. The main advantage of the SCEM-UA algorithm over existing approaches is that both calibration parameter values and associated uncertainties can be determined in a single optimization model run. In addition, no model linearity or parameter normality assumptions have to be made nor any derivatives calculated. The main drawback of the SCEM-UA methodology is that it could, potentially, be computationally demanding, although this is not envisaged as a major problem with current computers.     

Usefulness of molecular markers for detecting population bottlenecks via monitoring genetic change

It is important to detect population bottlenecks in threatened and managed species because bottlenecks can increase the risk of population extinction. Early detection is critical and can be facilitated by statistically powerful monitoring programs for detecting bottleneck-induced genetic change. We used Monte Carlo computer simulations to evaluate the power of the following tests for detecting genetic changes caused by a severe reduction in a population's effective size (Ne): a test for loss of heterozygosity, two tests for loss of alleles, two tests for change in the distribution of allele frequencies, and a test for small Ne based on variance in allele frequencies (the 'variance test'). The variance test was most powerful; it provided an 85% probability of detecting a bottleneck of size Ne = 10 when monitoring five microsatellite loci and sampling 30 individuals both before and one generation after the bottleneck. The variance test was almost 10-times more powerful than a commonly used test for loss of heterozygosity, and it allowed for detection of bottlenecks before 5% of a population's heterozygosity had been lost. The second most powerful tests were generally the tests for loss of alleles. However, these tests had reduced power for detecting genetic bottlenecks caused by skewed sex ratios. We provide guidelines for the number of loci and individuals needed to achieve high-power tests when monitoring via the variance test. We also illustrate how the variance test performs when monitoring loci that have widely different allele frequency distributions as observed in five wild populations of mountain sheep (Ovis canadensis).     

Pregnancies complicated by retained placenta: sex ratio and relation to pre-eclampsia

Pre-eclampsia and placenta accreta have opposite histological features of placentation. This study set out to test the hypotheses that the sex ratios in these two pregnancy complications are opposite and that these conditions are mutually exclusive. A population-based database covering all deliveries in South Australia between 1986 and 1995 and the hospital-based obstetric database of the Adelaide Women's and Children's Hospital, covering 8549 births between 1993 and 1995, were used to ascertain the sex ratios in singleton pregnancies and the sex ratios in those pregnancies in which there was retained placenta, hypertension in pregnancy, or pre-eclampsia. The likelihood of independence of occurrence or mutual exclusivity of retained placenta and hypertension in pregnancy or pre-eclampsia were also examined. The male:female sex ratio in the South Australian population was 1.077. In pregnancies with hypertension in pregnancy it was 1.165 (P<0.001) and in pregnancies with retained placenta it was 0.883 (P<0.0001). There was a trend to an increased sex ratio in pre-eclamptic pregnancy (1.248 in primigravid and 1.092 in multigravid women) but there was insufficient power to detect significance (P=0.207 and 0.470, respectively). Neither hypertension in pregnancy nor pre-eclampsia were mutually exclusive of placenta accreta: hypertensive disorders of pregnancy and placenta accreta occurred independently of each other. Our findings suggest that sex-linked antigens are unlikely to influence maternofetal interactions consistently to give rise to one but not the other pregnancy complication.     

Growth curve models for indistinguishable dyads using multilevel modeling and structural equation modeling: The case of adolescent twins' conflict with their mothers.

Growth modeling is a useful tool for studying change over time, and it is becoming increasingly popular with developmental researchers. There is a considerable methodological literature surrounding growth modeling for individuals; however, far less attention has been focused on growth models for pairs of related individuals (i.e., dyads). In this article, the authors consider dyadic growth models for those cases where there are no relevant variables that can empirically distinguish between dyad members (e.g., same-sex twins or best friends). The authors describe how researchers can estimate growth models for indistinguishable dyads using both multilevel modeling and structural equation modeling. Although both approaches can be used to estimate the same underlying models, the authors focus on practical similarities and differences between the two approaches. They illustrate modeling issues using an overtime study of adolescent twins' conflict with their mothers, a substantively important topic given the enduring interest in parent-child relationships during adolescence. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A cautionary note on the effects of range restriction on predictor intercorrelations.

The purpose of this research report is to highlight a unique set of issues that arise when considering the effects of range restriction in the context of estimating predictor intercorrelations. Three approaches are used to illustrate the issue: simulation, a concrete applied example, and a reanalysis of a meta-analysis of ability-interview correlations. The general conclusion is that a predictor intercorrelation can differ dramatically from the population value when both predictors are used in a composite that is used operationally for selection. The compensatory nature of a composite means that low scorers on one predictor can only obtain high scores on the composite if they obtain very high scores on the other predictor; this phenomenon distorts the correlation between the predictors. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Power of the independent samples t test under a prevalent psychometric measure distribution.

Studies in psychology often have low power because of inadequate sample size. Thus, recent articles in the Journal of Consulting and Clinical Psychology have suggested making sample size determinations through readily available tables that are based on population normality. Questions have been raised on the use of these power tables because prevalent psychometric distributions, such as the discrete mass at zero with gap that occurs with 1st use or onset variables, are radically nonnormal. In addition to demonstrating the robustness of the independent samples t test with respect to Type I error, the major finding of this study shows that researchers may use these power tables without modification for this radically nonnormal distribution. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Parental perspectives on chronic schizophrenia

Parents of chronic schizophrenics are an understudied population. Few researchers have asked them about their wants, needs, and difficulties. The little work that has been done indicates that their suffering is great, their coping skills better than had been previously assumed, and the stresses they are experiencing almost unendurable. This paper looks at what parents are coping with: a child with a terribly debilitating disease about which little is known, a culture which heaps stigma and shame upon them, very difficult and often unworkable mental health and legal systems, and to top it all off, a large number of clinical (non-researchers) professionals who are ignorant about schizophrenia and often unsympathetic towards the parents. This paper then looks at what little data we have from parents about their needs, and offers suggestions about how parents and professionals might best work together, towards the benefit of all concerned.     

Detecting infrequent deception.

Recent proposals for using the polygraph and similar devices in routine screening have been aimed at detecting deception in situations sometimes characterized by low base rates. Equations are developed that show that extraordinarily high levels of accuracy would be needed to detect infrequent deception. In this context, the debate over the accuracy of these methods is irrelevant; the accuracy needed to detect infrequent deception far exceeds the levels claimed by the most enthusiastic proponents of these detection techniques. The limits on the use of any particular test of deception can be determined by considering the base rate for deception and the proportion of the nondeceptive population that fails the test. When the base rate is less than .10, these limits are extremely restrictive. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Meta-analysis and moderator variables: A cautionary note on transportability.

Despite the recent popularity of meta-analysis as a tool for summarizing empirical results across a number of studies, surprisingly little research has been conducted on the accuracy of these procedures under a variety of population conditions. Of concern in this study was the 90% credibility value (K. Pearlman et al, see record 1980-31533-001) advocated as a rule of thumb regarding the transportability of employment test validities. We investigated the ability of this meta-analytic rule to detect the presence of discretely defined moderator variables, that is, the ability of the rule to detect instances where transportability is inappropriate. An infinite sample size analysis and a mathematical proof demonstrated that the transportability rule may produce erroneous inferences at rates higher than expected. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Justifying the use and increasing the power of a t test for a randomized experiment with a convenience sample.

The standard error in an independent-groups t test is usually derived assuming that individuals are randomly sampled from populations of infinite size. However, t tests are routinely used in randomized experiments with individuals obtained from convenience samples. In such cases, the use of a t test is most often justified by supposing (a) that the convenience sample was a random sample from a hypothetical infinite population and (b) that it is this hypothetical population to which inference is being drawn. It is shown how the use of a t test with a convenience sample can be justified without reference to a hypothetical infinite population and how it may be possible to modify the t test to increase its power for drawing inferences in randomized experiments. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Incidence of the prozone phenomenon in syphilis serology

OBJECTIVE: To determine the rate of the prozone phenomenon in our patient population. METHODS: Sera from 4328 patients--3504 females (2065 pregnant, 1439 nonpregnant) and 824 males--were tested for syphilis by the rapid plasma reagin (RPR) test, and then rechecked with serial twofold dilutions of up to 16-fold to detect the prozone phenomenon. Chi-square analysis with Yates correction was used, with P < or = .05 considered significant. RESULTS: The total positivity rate in females was 6% (213 patients); 13% (27) of the positive tests were false-positive reactions as confirmed by a negative anti-treponemal antibody test. Only one prozone reaction was detected, in serum from a male subject, but it was not missed on initial screening. CONCLUSION: The rate of prozone phenomenon is very low (95% confidence interval 0-0.4%), and routine serial dilutions are not cost effective.     

Power and sample size calculations for studies involving linear regression

This article presents methods for sample size and power calculations for studies involving linear regression. These approaches are applicable to clinical trials designed to detect a regression slope of a given magnitude or to studies that test whether the slopes or intercepts of two independent regression lines differ by a given amount. The investigator may either specify the values of the independent (x) variable(s) of the regression line(s) or determine them observationally when the study is performed. In the latter case, the investigator must estimate the standard deviation(s) of the independent variable(s). This study gives examples using this method for both experimental and observational study designs. Cohen's method of power calculations for multiple linear regression models is also discussed and contrasted with the methods of this study. We have posted a computer program to perform these and other sample size calculations on the Internet (see http://www.mc.vanderbilt.edu/prevmed/psintro+ ++.htm). This program can determine the sample size needed to detect a specified alternative hypothesis with the required power, the power with which a specific alternative hypothesis can be detected with a given sample size, or the specific alternative hypotheses that can be detected with a given power and sample size. Context-specific help messages available on request make the use of this software largely self-explanatory.     

EBP ≠ EST: Reply to Crits-Christoph et al. (2005) and Weisz et al. (2005).

Replies to Crits-Christoph et al. (see record 2005-04167-007) and Weisz et al (see record 2005-04167-008). Evidence-based practice (EBP) is not a synonym of empirically supported therapies (ESTs). ESTs reflect 1 kind of evidence that should guide EBP. The authors focus in this response on 4 issues: the distinction between 2 functions of randomized clinical trials (RCTs) with very different methodological and clinical implications, problems with the experimental designs used to test ESTs that have unnecessarily limited their clinical and scientific utility, the question of how to integrate findings from RCTs with findings from other empirical methods to guide evidence-based interventions, and what one means by clinicians (i.e., whether clinicians should be paraprofessionals who implement procedures developed by researchers or full-fledged partners in the development and implementation of evidence-based psychological practice). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Need-based resource allocation--experiences with the RAWP formula in Great Britain

The RAWP formula used for resource allocation in Great Britain between 1976 and 1991 is a morbidity-oriented instrument of controlling, which has so far received only little attention in Germany. The development of this model was supported by the intention to intervene in the regional pattern of hospital supply by means of resource allocation and to refine it according to the guiding principles of equity and efficiency. The basic elements-regional population, average bed use, ICD chapter-specific SMRs-are discussed and the various modifications outlined. The RAWP formula's potentials of controlling resulted in a progressive reduction of the apparent disparities between regions in hospital supply, and knee was considered to be a "qualified success". The future development in the sense of an internal market addressed.     

Revisiting the picture-superiority effect in symbolic comparisons: Do pictures provide privileged access?

1n 4 experiments, symbolic comparisons were investigated to test semantic-memory retrieval accounts espousing processing advantages for picture over word stimuli. In Experiment 1, participants judged pairs of animal names or pictures by responding to questions probing concrete or abstract attributes (texture or size, ferocity or intelligence). Per pair, attributes were salient or nonsalient concerning their prerated relevance to animals being compared. Distance (near or far) between attribute magnitudes was also varied. Pictures did not significantly speed responding relative to words across all other variables. Advantages were found for far attribute magnitudes (i.e., the distance effect) and salient attributes. The distance effect was much less for salient than nonsalient concrete-attribute comparisons. These results were consistently found in additional experiments with increased statistical power to detect modality effects. Our findings argue against dual-coding and some common-code accounts of conceptual attribute processing, urging reexamination of the assumption that pictures confer privileged access to long-term knowledge. (PsycINFO Database Record (c) 2010 APA, all rights reserved)