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The power of the transmission disequilibrium test (TDT) with both case-parent and control-parent trios.

The transmission disequilibrium test (TDT) customarily uses affected children and their parents (often case-parent trios, TDTD). Control-parent trios are necessary to guard against spurious significant results due to segregation distortion but are not generally utilized in the identification of disease susceptibility loci (DSL). Controls are often easy to recruit and the TDT can easily be extended to include control-parent trios into the analyses with unrelated case-parent trios. We present an extension of the TDT (TDTDC) that incorporates unrelated cases and controls and their parents into a single analysis. We develop a simple and accurate analytical method for computing the statistical power of various TDT (e.g. the TDTD, TDTDC, TDTDC and TDTC that employ control-parent trios only) under any genetic model. We investigated the power of these TDT, and particularly compared the relative power of the TDTD and TDTDC. We found that the TDTDC is almost always more powerful than the TDTC and TDTD. The relative power of the TDTDC and TDTD depends largely upon a number of parameters identified in the study. This study provides a basis for efficient use of control-parent trios in DSL identification.

Case-Control Studies↗

Is it necessary to transform nutrient variables prior to statistical analyses?

The distributions of the intakes of many nutrients are skewed, yet this is often overlooked when standard statistical analyses are applied to nutrient data. The nutrient intakes of 5,123 men and 5,236 women, recorded by food frequency questionnaire in the Scottish Heart Health Study, were transformed to achieve approximately symmetric distributions. Power transformations were chosen using letter value analyses. A letter value analysis uses selected order statistics and their position around the median to assess symmetry. The effect that each transformation had on a comparison of nutrient intakes between those with and without prevalent coronary heart disease was determined from t tests on the untransformed and transformed variable. The effects of the logarithm and square root transformation and of the optimum Box-Cox transformation were also determined, and the results were compared with the nonparametric Mann-Whitney test. The conclusion of whether or not to reject the null hypotheses often varied, depending on the transformation and test used. The nonparametric test usually gave a conclusion similar to that of the t test on the letter value-transformed data, the Box-Cox-transformed variable, and after either the logarithm or square root transformation of the data, but not always both. The results from the untransformed variable were sometimes very different. Failure to account for skewness in nutrient variables may thus lead to spurious conclusions.

Aged↗

Effect of brushing with sonic and counterrotational toothbrushes on the bond strength of orthodontic brackets.

The purpose of this study was to evaluate the effect of brushing with both a sonic and mechanical counter rotary power toothbrush on the bond strength of orthodontic brackets. Forty-five extracted teeth were divided into three random groups and had orthodontic brackets bonded to them. One group was brushed with a counterrotational toothbrush, the Interplak, one group with a sonic toothbrush, the Sonicare, and a third group was not brushed and was held as a control. After the equivalent of 2 years brushing, the teeth were placed in an Instron machine and the shear force to remove the brackets was recorded. Group 1, the counter rotary power brush, had a mean of 107.5 kg/cm2, the second group, the sonic brush, had a mean of 79.7 kg/cm2, and the control group had a mean of 125. 4 kg/cm2. Single factor analysis of variance followed by the Fisher-Hayter Multiple Comparison Procedure showed a statistically significant difference between the sonic power brush and the control (P <.01), but no significant difference between the counter rotary and the control (P >.05). There was no significant difference between the two power brushes (P >.05).

Analysis of Variance↗

Antibodies to oxidized low-density lipoproteins and angiographically assessed coronary artery disease in white patients.

BACKGROUND: Low-density lipoprotein (LDL) can be oxidatively modified by reactive oxygen species, thus generating oxLDL. The latter induce formation of specific antibodies (oxLDLAb), which are detectable in patients with atherosclerosis, in which they might play a pathogenic or a protective role. Thus, we aimed to investigate the association of antibodies with oxidized LDLs (oxLDL) (oxLDLAbs) with coronary artery disease (CAD) and acute coronary syndromes. METHODS AND RESULTS: In a cross-sectional study of 529 consecutive patients undergoing quantitative coronary angiography for suspected CAD, we measured the titer of IgG oxLDLAbs by ELISA. With regression analysis techniques, we also investigated the determinants of oxLDLAb titer and the association of oxLDLAbs with CAD severity. We found no significant differences of oxLDLAb titer between groups of patients without and with different CAD severity. The oxLDLAb titer was 18.6 enzyme units (EU) (11.5 to 25.7 EU/mL) (mean, 95% CI) in patients without CAD; 16.8 EU (9.6 to 24.2 EU) in patients with stenosis <50%; and 19.9 EU (15 to 24.8 EU), 17.2 (13.8 to 20.6 EU), and 14.7 EU (12.1 to 17.3 EU) in those with in 1-, 2-, or 3-vessel > or =50% stenosis, respectively. Similarly, no differences of oxLDLAb titer between patients without and with acute coronary syndrome were found. The oxLDLAb titer correlated weakly with aging and with serum total, LDL, and HDL cholesterol and plasma homocysteine levels; however, only age and HDL cholesterol remained significant predictors of the oxLDLAb titer at a stepwise regression analysis. CONCLUSIONS: The results of this study, which was adequately powered from the statistical standpoint, provided no evidence for an association of IgG oxLDLAb titer with angiographically assessed CAD in whites.

Age Factors↗

Introduction to statistical analysis of population data in immunogenetics.

We review the most classical questions addressed by the analysis of population data in immunogenetics. Basic genetics' definitions are reminded. Questions related to the population data itself (structure, missing values nomenclature, sampling) are developed first, and secondly, the population genetics questions (relevance of genetic parameters (phenotype, genotype allele and haplotype frequencies; genetic distances; linkage disequilibrium measures), methods and practical computing) are illustrated by immunogenetics polymorphisms. This article gives the essential of population immunogenetics on examples and key references. We underline the importance of population dimension in statistical analysis: structure of linkage disequilibrium and genetic diversity between populations may affect the power of the study or the interpretation of correlations between markers, genes and diseases, making population genetics both theoretical and very practical.

Data Interpretation, Statistical↗

Multivariate linkage analysis of blood pressure and body mass index.

Multivariate linkage analyses of correlated traits provide greater statistical power to identify genetic loci with effects too small to be detected in single trait analyses. We conducted genomewide multivariate analyses of systolic BP, diastolic BP, and body mass index (BMI) in 1,848 non-Hispanic white subjects (968 females, 880 males) from 279 multigenerational pedigrees from Rochester, Minnesota. Blood pressure was measured by random zero sphygmomanometer; body mass index was calculated from measurements of height and weight; and genotypes were measured at 520 microsatellite marker loci distributed across the 22 autosomes. Univariate linkage analyses demonstrated tentative evidence of linkage (defined by univariate LOD scores of 1.30-1.99) for diastolic BP on chromosome 18 and for BMI on chromosomes 3, 10, and 18. Bivariate linkage analyses showed tentative evidence of linkage (defined by bivariate LOD scores of 2.06-2.86) for systolic and diastolic BP on chromosome 14 and for either measure of BP and BMI on chromosomes 2, 3, 10, and 18; and suggestive evidence of linkage (defined by bivariate LOD scores of 2.87-3.99) for either measure of BP and BMI on chromosomes 10 and chromosome 15. Trivariate linkage analyses of systolic and diastolic BP and BMI provided evidence of a region influencing all three traits on chromosome 10, where the trivariate LOD score rose to a maximum value of 4.09 (at 144 cM, P=0.0007), and possibly on chromosome 2, where it rose to a maximum value of 2.80 (at 77 cM, P=0.0075). For genomewide linkage analyses to succeed in localizing genes influencing BP, it may be advantageous to exploit the greater statistical power of multivariate linkage analyses to identify loci with pleiotropic effects on correlated traits.

Adult↗

Computer analysis of the electroencephalographic activity of the Caiman olfactory bulb.

Bipolar EEG records of olfactory activity of Caiman sclerops were computer analyzed and the derived power spectral density functions (PSD) were statistically compared by the method of Bendat and Piersol (1971) and Fails and Verlander (1977). These recordings included surface and underwater episodes. The Caiman olfactory EEG may legitimately be divided into four types of activity: a spindle form of high amplitude and long duration (1-3 sec), a spindle of lower amplitude, short duration (less than 1 sec) and significantly lower power generation irregular surface background activity, and an irregular underwater activity. PSD analysis showed a great deal of moment-to-moment variability in both quantity of power generated and the frequency regions containing significant power generation during surface background activity. A statistical comparison of surface background and underwater background activity also showed significant differences.

Animals↗

Effect of continuous versus dichotomous outcome variables on study power when sample sizes of orthopaedic randomized trials are small.

It is often not feasible to conduct large trials in orthopaedic surgery. Therefore, surgeons must identify strategies to optimize the statistical power of their smaller studies. The aim of this study was to compare study power in randomized trials with continuous versus dichotomous outcome variables. We performed a systematic review of the literature to identify randomized trials in orthopaedic trauma. Of these, we examined only those trials with small sample sizes (50 patients or less). The outcomes in each eligible study were categorized as continuous or dichotomous. Standard power calculations were performed for each study, and comparisons were made between continuous and dichotomous outcome variables. We identified 196 randomized trials in orthopaedic trauma. Of these, 76 trials had a sample size of 50 patients or fewer (29 trials with continuous outcomes, 47 trials with dichotomous outcomes). Studies that reported continuous outcomes had a significantly higher mean power than those that reported dichotomous variables (power 49% vs 38%, p=0.042). Twice as many trials with continuous outcome variables reached acceptable levels of study power (i.e. >80% power) when compared with trials with dichotomous variables (37% vs 18.6%, p=0.04). When orthopaedic surgeons anticipate small sample sizes for their study, they can optimize their study's statistical power by choosing a continuous outcome variable.

Cluster Analysis↗

A comparison of summary patient-level covariates in meta-regression with individual patient data meta-analysis.

OBJECTIVES: To compare meta-analysis of summary study level data with the equivalent individual patient data (IPD) analysis when interest lies in identification of binary patient characteristics related to treatment efficacy. DESIGN: A simulation study comparing meta-regression with IPD analyses of randomized controlled trials. METHODS: Twenty-seven different meta-analysis situations were simulated with 1000 repetitions in each case. The following parameters were varied: (1) the treatment effect magnitude for different patient risk groups; (2) sample sizes of individual studies; and (3) number of studies. The meta-regression and IPD results were then compared for each situation. RESULTS: The statistical power of meta-regression was dramatically and consistently lower than that of IPD analysis, with little agreement between the parameter estimates obtained from the two methods. Only in meta-analyses of large numbers of large trials, did meta-regression detect differential treatment effects between risk groups with any consistency. CONCLUSIONS: Meta-analysis of summary data may be adequate when estimating a single pooled treatment effect or investigating study level characteristics. However, when interest lies in investigating whether patient characteristics are related to treatment, IPD analysis will generally be necessary to discover any such relationships. In these situations practitioners should try to obtain individual-level data.

Humans↗

[Changes with age in EEG during anesthesia].

The number of older persons who have to undergo surgical procedures is steadily growing. For these patients the risks of anaesthesia are often increased because of their past medical history and their restricted physiological resources. Apart from parameters of the cardiovascular system, the electroencephalogram (EEG) represents a supplementary method for intraoperative monitoring, because cerebral alterations caused by anaesthetics or narcotics are directly reflected in the EEG. In routinely conducted registrations of the EEG in the operating theatre it appeared that the EEG of older persons differed from the EEG of younger patients. The aim of the present study was to further investigate the effect of patients' age on the EEG during anaesthesia. METHODS. Three data sets from different EEG registrations were analysed. The first data set consisted of inductions of anaesthesia with 7 mg/kg body weight thiopental in 43 patients from 17 to 80 years of age (mean 53.6 +/- 16.7 years) using derivations C3-P3 and Cz-A1. The second data set included 69 EEG registrations of general anaesthesia induced with barbiturates and maintained with enflurane in patients from 16 to 83 years (mean 51.4 +/- 17.7 years). The third data set comprised inductions of anaesthesia with 2 mg/kg body weight propofol. EEGs of the second and third data set were recorded with the EEG monitor 'Narkograph' using derivation C3-P3 and derivations C3-P3/C4-P4, respectively. Classification of the EEGs was performed according to the proposals of Kugler [12]. The basis for the statistical analysis of all data sets was formed by parameters from the power spectra of the EEG recordings. RESULTS. The data from inductions of anaesthesia with thiopental and propofol showed EEG patterns from alpha-EEG to burst suppression activity, whereby periods with burst suppressions could more often be observed in the EEG of older people. Under thiopental burst suppression activity occurred in 20% of patients up to 50 years, in 47% of those between 50 and 70 years and in 89% over 70 years. The corresponding figures for propofol were 0%, 5% and 54%, respectively. Figure 2 depicts the correlation between age and power for the thiopental data. The power decreases with increasing age of the patients. This result led to further investigations of the effect of patients' age on the power in different EEG stages. Of special interest were deep stages of anaesthesia, because especially in these stages visual inspections revealed smaller amplitudes of the EEG signal for older patients than for younger persons. Figure 3 shows the power in the delta frequency band in deep stages of barbiturate-induced enflurane anaesthesia for patients of different age groups. The power in the delta frequency band distinctly decreases for geriatric patients. The same effect could be observed for the propofol data (Fig. 4). CONCLUSIONS. The EEG represents an important method for effective intraoperative monitoring and contributes to an individually adjusted course of anaesthesia, especially for geriatric patients. In these patients, clinical signs such as parameters of the cardiovascular system, which are usually used to judge the depth of anaesthesia, are often altered by the patient's past medical history or by drugs. Furthermore, geriatric patients show a reduced need for narcotic agents. However, the variation of the required dosage is greater in older than in younger persons. The results of the present study show that with regard to an automatic classification of the EEG during anaesthesia, alterations of the EEG with age have to be taken into account.

Adolescent↗

A randomized, open-label pharmacokinetic comparison of two oral formulations of fluconazole 150 mg in healthy adult volunteers.

BACKGROUND: Because of its systemic action, fluconazole is prescribed for a variety of fungal infections. However, therapeutic failure might result when a patient is switched between an innovator drug and a nonbioequivalent generic formulation. Pharmacokinetic (PK) studies investigating the bioequivalence of generic and innovator drugs can minimize such risks. OBJECTIVE: The aim of this study was to compare the PK profiles and relative bioavailabilities of 2 oral formulations of fluconazole: Diflucan (reference; Pfizer Corporation Austria GmbH, Wien, Austria) and Funzol (test; Bosnalijek d.d., Pharmaceutical and Chemical Industry, Sarajevo, Bosnia and Herzegovina), both prepared as capsules containing 150 mg of active drug. METHODS: A single oral dose of fluconazole was given under fasting conditions to healthy, white volunteers aged 18 to 55 years in this open-label, randomized, crossover study. A 3-week washout period was applied between each of the 2 doses. Serum samples were obtained before dosing and at various time points after dosing up to 144 hours and were analyzed for fluconazole concentration using a high-performance liquid chromatography-UV method. PK parameters representing the extent (AUC(0-infinity)) and rate (CmaX and T(max)) of absorption of fluconazole were obtained. An analysis of variance, a power analysis, 90% CI, and two 1-sided tests were used for statistical analysis of relative differences between the 2 drugs. Bioequivalence was concluded if the 90% CIs for the geometric mean ratios of AUC(0-infinity) and C(max) were between 0.80 and 1.25. A study investigator monitored the volunteers for adverse effects at 5 defined time points during the clinical part of the investigation. RESULTS: Thirteen men and 11 women (mean age, 33.3 years; mean weight, 73.6 kg) completed the study. The respective point estimates of the ratios of geometric means of log-transformed C(max) and AUC0(0-infinity) of fluconazole (test vs reference) were 0.985 and 1.047, with 90% CIs of 0.894 to 1.085 and 0.927 to 1.182, respectively. Differences in T(max) also did not reach statistical significance. No adverse effects were reported by the subjects or revealed by clinical or laboratory tests. CONCLUSIONS: The study failed to demonstrate any statistically significant differences in C(max) and AUCO(0-infinity) values between the test and reference formulations of oral fluconazole 150 mg in this small, select population of healthy volunteers. On that basis, and according to both the rate and extent of absorption, the test and reference formulations were considered bioequivalent.

Administration, Oral↗

Therapeutic decision tree for patients with sustained ventricular tachyarrhythmias or aborted cardiac arrest: a critical review of the Antiarrhythmics Versus Implantable Defibrillator trial and the Canadian Implantable Defibrillator Study.

Antiarrhythmic drugs, mainly amiodarone and sotalol, radiofrequency catheter ablation, and the implantable cardioverter defibrillator (ICD) are the 3 therapeutic options in patients with sustained ventricular tachycardia (VT) or ventricular fibrillation (VF). Idiopathic VT, incessant VT, frequently recurring, hemodynamically stable VT, and VT based on bundle branch reentry, are candidates for radiofrequency catheter ablation. Patients with high-risk ventricular tachyarrhythmias should receive ICDs as initial therapy. Two studies, the Antiarrhythmics Versus Implantable Defibrillator trial (AVID) and the Canadian Implantable Defibrillator Study (CIDS) have tried to approach the problem of these high-risk ventricular tachyarrhythmias. Although at 3 years, the ICD in AVID demonstrated a significant relative risk reduction over amiodarone of 31.5%, CIDS could not duplicate this finding. At 3 years, the relative risk reduction conferred by the ICD over amiodarone in CIDS was only 13.7%. A careful analysis of both studies suggests that CIDS was insufficiently powered to demonstrate statistically significant benefits similar to those shown by AVID, and furthermore, seemed to include an undetermined number of low-risk VT patients. The problem in the CIDS trial in this regard was the recruitment of patients in whom the inclusion criteria were met by the arrhythmias induced during the electrophysiology stimulation study, but which did not exist in real life. In addition CIDS included 14% of patients with (1) undocumented syncope and inducible monomorphic sustained VT; or (2) long runs of spontaneous nonsustained VT. Under these circumstances, the therapeutic implications of AVID remain unchallenged.

Anti-Arrhythmia Agents↗

Variance component methods for detecting complex trait loci.

Variance component-based linkage analysis has become a major statistical tool for the localization and evaluation of quantitative trait loci influencing complex phenotypes. The variance component approach has many benefits--it can, for example, be used to analyze large pedigrees, and it is able to accommodate multiple loci simultaneously in a true oligogenic model. Important biological phenomena such as genotype-environment interaction and epistasis are also examined easily in a variance component framework. In this chapter, we review the basic statistical features of variance component linkage analysis, with an emphasis on its power and robustness to distributional violations.

Analysis of Variance↗

Randomized trials of high dose chemotherapy for breast cancer.

'Now is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning'Winston Churchill in a speech to the Canadian Senate and House of Commons, December 30, 1941. In laboratory models of cancer, dose of cytotoxic chemotherapy correlates with curative therapy, while cumulative dose is associated with longer survival for those who are not cured. These observations suggests a strategy of using high doses when cure is the objective but smaller doses over a prolonged period when palliation and survival are the goal. A strategy combining repetitive cycles of higher doses of cytotoxic therapy, followed by the optimal combination of hormonal and biological agents based on the tumor's receptors might contribute to both the highest possible cure rate and the longest survival. The development of bone marrow transplant (BMT) for leukemias, and its subsequent modification for support after high dose therapy for other malignancies, has a long, complex and emotional history in medicine. At least partly because of firmly held opinions and the way large randomized trials are funded in the United States, few American randomized trials of BMT or high dose therapy strategies have been completed. The vast majority of published randomized BMT and high dose studies are European. Interestingly, in contrast, two large American randomized trials of high dose chemotherapy for breast cancer had actually completed accrual. Accrual on a third was on target until the presentation of five very small or very early randomized trials at the American Society of Clinical Oncology meeting in May of 1999. Results from some of these trials, which were analyzed after a relatively brief follow-up, are too premature to allow definitive conclusions. Nevertheless, these data have been over and misinterpreted within the scientific and lay communities. The remaining studies included a limited number of patients, thus restricting the statistical power of the observations. The desire for quick answers impeded dispassionate analysis of the available data. The opportunity for collegial review of the data further deteriorated with another round of press coverage when the data from the South African adjuvant study were found to be unreliable. Rather than increasing commitment to accrual on randomized and appropriate pilot trials, accrual to the only large American study in existence at that time trickled to a halt. In response to press coverage, Susan Edmonds from the Fred Hutchinson Cancer Research Center observed that 'the NYT article tends to cast shadows generally on the therapy and those providing the therapy rather than pointing out early in the article (where the public will readily see it) that there are a number of very credible research institutions conducting research directed at breast cancer, some looking at high dose chemotherapy and stem cell transplantation.' Dr. Rodenhuis, presenting the large positive Dutch Randomized study (funded by the Dutch insurance industry) at ASCO in 2000, commented on the 'unreasonably high expectations until 1999' and 'unreasonably negative [opinion-ed] since 1999' for high dose adjuvant chemotherapy for breast cancer.

Antineoplastic Combined Chemotherapy Protocols↗

Utility of B-type natriuretic peptide in the diagnosis of congestive heart failure in an urgent-care setting.

OBJECTIVES: The goal of this study was to evaluate the utility of a rapid "bedside" technique for measurement of B-type natriuretic peptide (BNP) in the diagnosis of congestive heart failure (CHF) in an urgent-care setting. BACKGROUND: B-type natriuretic peptide is a protein secreted from the cardiac ventricles in response to pressure overload. One potential application of measurements of BNP in blood is distinguishing dyspnea due to CHF from other causes. METHODS: B-type natriuretic peptide concentrations were measured in a convenience sample of 250 predominantly male (94%) patients presenting to urgent-care and emergency departments of an academic Veteran's Affairs hospital with dyspnea. Results were withheld from clinicians. Two cardiologists retrospectively reviewed clinical data (blinded to BNP measurements) and reached a consensus opinion on the cause of the patient's symptoms. This gold standard was used to evaluate the diagnostic performance of the BNP test. RESULTS: The mean BNP concentration in the blood of patients with CHF (n = 97) was higher than it was in patients without (1,076 +/- 138 pg/ml vs. 38 +/- 4 pg/ml, p < 0.001). At a blood concentration of 80 pg/ml, BNP was an accurate predictor of the presence of CHF (95%); measurements less than this had a high negative predictive value (98%). The overall C-statistic was 0.97. In multivariate analysis, BNP measurements added significant, independent explanatory power to other clinical variables in models predicting which patients had CHF. The availability of BNP measurements could have potentially corrected 29 of the 30 diagnoses missed by urgent-care physicians. CONCLUSIONS: B-type natriuretic peptide blood concentration measurement appears to be a sensitive and specific test to diagnose CHF in urgent-care settings.

Aged↗

Combined modality therapy of esophageal cancer--standard of care?

Because of the controversies and conflicting data described above, it is very difficult to provide strict guidelines for the treatment of patients with locoregional esophageal carcinoma. It is necessary, however, to try to put current knowledge in perspective. Surgery remains a standard of care for potentially resectable disease. Definitive chemoradiation is a standard of care for locoregional disease, particularly if a patient is medically unfit for surgery, if a surgeon experienced in esophagectomies is unavailable, or if the patient has cervical disease, which would require very extensive surgery. Chemoradiation is superior to radiation alone for these patients. Some frail patients may only be able to tolerate palliative radiation. Preoperative chemotherapy is controversial, with two large randomized studies resulting in two different conclusions regarding survival benefit. NCCN guidelines do not suggest this modality as a standard of care. After assessment of the patient's tumor stage, performance status, and personal wishes for aggressive treatment, it is possible that some physicians may consider offering this option to patients. The patient should have advanced disease, an excellent performance status, and understand the controversies and lack of consensus about this option. Preoperative chemoradiation is controversial. Only one of three randomized trials showed a clear survival benefit; however, the patients treated with surgery alone in that trial had an unusually poor outcome. The second trial showed no benefit, and the third trial was negative but statistically powered only to reveal a large survival difference and in multivariate analysis showed a possible trend to improved outcome. The NCCN guidelines do not advocate preoperative chemoradiation as a standard of care. As stated above, however, some physicians may consider offering this option to patients with advanced disease, excellent performance status, and who understand the controversies and lack of consensus about this option. Patients with adenocarcinoma of the GE junction treated with surgery alone should have postoperative chemoradiation, particularly if there is tumor involvement in the lymph nodes. No definitive trial has been performed to determine whether chemoradiation plus surgery is superior to chemoradiation alone.

Combined Modality Therapy↗

A method for normalizing drug responses to enhance reliability of parametric statistical tests.

Experimental constraints often require that pharmacologists study the effects of treatments upon responses elicited with a single concentration of agonist. The choice of statistical analysis, nonparametric or parametric, will depend upon whether or not the responses are normally distributed. This study uses both Monte Carlo and theoretical approaches to examine the normality assumption as it applies to drug responses. Responses measured in systems where drug sensitivities follow a lognormal distribution are generally not normally distributed. A simple transformation of responses, however, can restore normality. Researchers may find it beneficial to transform response data prior to performing t tests, analysis of variance or other parametric statistics in order to preserve the power and confidence level of the test.

Dose-Response Relationship, Drug↗

DF-analyses of heritability with double-entry twin data: asymptotic standard errors and efficient estimation.

In this paper we establish the asymptotic distribution of DeFries Fulker (1985) regression estimates for heritability and shared environmental influences with double-entry twin data. A simple formula to estimate the covariance matrix of the coefficients in DF-regressions is provided, and applications to simulated data and Danish twin data show that this method can substantially increase the statistical power of the analyses. We also provide an 'efficient DF-analysis' that yields more precise estimates when additional covariates are included among the explanatory variables.

Adult↗