A power primer

One possible reason for the continued neglect of statistical power analysis in research in the behavioral sciences is the inaccessibility of or difficulty with the standard material. A convenient, although not comprehensive, presentation of required sample sizes is provided here. Effect-size indexes and conventional values for these are given for operationally defined small, medium, and large effects. The sample sizes necessary for .80 power to detect effects at these levels are tabled for eight standard statistical tests: (a) the difference between independent means, (b) the significance of a product-moment correlation, (c) the difference between independent rs, (d) the sign test, (e) the difference between independent proportions, (f) chi-square tests for goodness of fit and contingency tables, (g) one-way analysis of variance, and (h) the significance of a multiple or multiple partial correlation.

A power primer

One possible reason for the continued neglect of statistical power analysis in research in the behavioral sciences is the inaccessibility of or difficulty with the standard material. A convenient, although not comprehensive, presentation of required sample sizes is provided here. Effect-size indexes and conventional values for these are given for operationally defined small, medium, and large effects. The sample sizes necessary for .80 power to detect effects at these levels are tabled for eight standard statistical tests: (a) the difference between independent means, (b) the significance of a product-moment correlation, (c) the difference between independent rs, (d) the sign test, (e) the difference between independent proportions, (f) chi-square tests for goodness of fit and contingency tables, (g) one-way analysis of variance, and (h) the significance of a multiple or multiple partial correlation.

The global assessment scale. A procedure for measuring overall severity of psychiatric disturbance

The Global Assessment Scale (GAS) is a rating scale for evaluating the overall functioning of a subject during a specified time period on a continuum from psychological or psychiatric sickness to health. In five studies encompassing the range of population to which measures of overall severity of illness are likely to be applied, the GAS was found to have good reliability. GAS ratings were found to have a greater sensitivity to change over time than did other ratings of overall severity or specific symptom dimensions. Former inpatients in the community with a GAS rating below 40 had a higher probability of readmission to the hospital than did patients with higher GAS scores. The relative simplicity, reliability, and validity of the GAS suggests that it would be useful in a wide variety of clinical and research settings.

The global threat of antimicrobial resistance: science for intervention

In the last decade we have witnessed a dramatic increase in the proportion and absolute number of bacterial pathogens resistant to multiple antibacterial agents. Multidrug-resistant bacteria are currently considered as an emergent global disease and a major public health problem. The B-Debate meeting brought together renowned experts representing the main stakeholders (i.e. policy makers, public health authorities, regulatory agencies, pharmaceutical companies and the scientific community at large) to review the global threat of antibiotic resistance and come up with a coordinated set of strategies to fight antimicrobial resistance in a multifaceted approach. We summarize the views of the B-Debate participants regarding the current situation of antimicrobial resistance in animals and the food chain, within the community and the healthcare setting as well as the role of the environment and the development of novel diagnostic and therapeutic strategies, providing expert recommendations to tackle the global threat of antimicrobial resistance.

An epidemiological study of disorders in late childhood and adolescence--I. Age- and gender-specific prevalence

Developmental aspects of psychiatric disorders may be inferred from patterns of age differences in prevalence. Age-specific prevalences are provided for nine disorders in a general population sample of ages 10-20. Age and gender patterns for several disorders suggest developmental stage-associated risks. These include oppositional disorder in both genders and conduct disorder and major depression in girls. Major depression shows a pattern suggestive of a role for the onset of puberty. The prevalence of one or more disorders did not differ by age or gender. However, the pattern of specific diagnoses varied greatly by both age and gender.

Role of locus coeruleus in attention and behavioral flexibility

Previous findings have implicated the noradrenergic locus coeruleus (LC) system in functions along the dimension of arousal or attention. It has remained uncertain what role this system has in attention, or what mechanisms may be involved. We review our recent work examining activity of LC neurons in monkeys performing a visual discrimination task that requires focused attention. Results indicate that LC cells exhibit phasic or tonic modes of activity, that closely correspond to good or poor performance on this task, respectively. A computational model was used to simulate these results. This model predicts that alterations in electrotonic coupling among LC cells may produce the different modes of activity and corresponding differences in performance. This model also indicates that the phasic mode of LC activity may promote focused or selective attention, whereas the tonic mode may produce a state of high behavioral flexibility or scanning attentiveness. The implications of these results for clinical disorders such as attention-deficit hyperactivity disorder, stress disorders, and emotional and affective disorders are discussed.

Long-term weight loss and changes in blood pressure: results of the Trials of Hypertension Prevention, phase II

BACKGROUND

Weight loss appears to be an effective method for primary prevention of hypertension. However, the long-term effects of weight loss on blood pressure have not been extensively studied.

OBJECTIVE

To present detailed results from the weight loss arm of Trials of Hypertension Prevention (TOHP) II.

DESIGN

Multicenter, randomized dinical trial testing the efficacy of lifestyle interventions for reducing blood pressure over 3 to 4 years. Participants in TOHP II were randomly assigned to one of four groups. This report focuses only on participants assigned to the weight loss (n = 595) and usual care control (n = 596) groups.

PATIENTS

Men and women 30 to 54 years of age who had nonmedicated diastolic blood pressure of 83 to 89 mm Hg and systolic blood pressure less than 140 mm Hg and were 110% to 165% of their ideal body weight at baseline.

INTERVENTION

The weight loss intervention included a 3-year program of group meetings and individual counseling focused on dietary change, physical activity, and social support

MEASUREMENTS

Weight and blood pressure data were collected every 6 months by staff who were blinded to treatment assignment

RESULTS

Mean weight change from baseline in the intervention group was -4.4 kg at 6 months, -2.0 kg at 18 months, and -0.2 kg at 36 months. Mean weight change in the control group at the same time points was 0.1, 0.7, and 1.8 kg. Blood pressure was significantly lower in the intervention group than in the control group at 6, 18, and 36 months. The risk ratio for hypertension in the intervention group was 0.58 (95% CI, 0.36 to 0.94) at 6 months, 0.78 (CI, 0.62 to 1.00) at 18 months, and 0.81 (CI, 0.70 to 0.95) at 36 months. In subgroup analyses, intervention participants who lost at least 4.5 kg at 6 months and maintained this weight reduction for the next 30 months had the greatest reduction in blood pressure and a relative risk for hypertension of 0.35 (CI, 0.20 to 0.59).

CONCLUSIONS

Clinically significant long-term reductions in blood pressure and reduced risk for hypertension can be achieved with even modest weight loss.

A clinical and pathological staging system for soft tissue sarcomas

A system for staging the clinical status of patients with soft tissue sarcomas is presented, based on the clinical characteristics of the primary tumor (size, extension), the involvement of lymph nodes, the presence of metastases, and the grade of the tumor. This represents the TNM system with grade of tumor (G) added. The system evolved was based on examination of 1215 cases of 13 types of soft tissues sarcomas, primarily in the extremities (fibrosarcoma, liposarcoma, etc.). Nine stages are described, and they are correlated with survival in the cases reviewed. The staging system now can be used for case evaluation for therapy determination and for intercomparison of series of patients as to incidence of different kinds of tumors, effects of treatment, and survival.

Sympathetic skin response--a method of assessing unmyelinated axon dysfunction in peripheral neuropathies

The sympathetic skin response (SSR) was measured in 33 patients with peripheral neuropathies and in 30 normal control subjects. Abnormalities of the response were correlated with clinical, pathologic, and EMG observations. The response was usually absent in axonal neuropathies, but present in demyelinating disorders. Abnormalities of the sympathetic skin response did not correlate well with clinical evidence of dysautonomia, but were a reliable indicator of disorders affecting unmyelinated axons.

Fertility and ageing

The late 20th century trend to delay birth of the first child until the age at which female fecundity or reproductive capacity is lower has increased the incidence of age-related infertility. The trend and its consequences have also stimulated interest in the possible factors in the female and the male that may contribute to the decline in fecundity with age; in the means that exist to predict fecundity; and in the consequences for pregnancy and childbirth. In the female, the number of oocytes decreases with age until the menopause. Oocyte quality also diminishes, due in part to increased aneuploidy because of factors such as changes in spindle integrity. Although older male age affects the likelihood of conception, abnormalities in sperm chromosomes and in some components of the semen analysis are less important than the frequency of intercourse. Age is as accurate as any other predictor of conception with assisted reproductive technology. The decline in fecundity becomes clinically relevant when women reach their mid-30s, when even assisted reproduction treatment cannot compensate for the decline in fecundity associated with delaying attempts at conceiving. Pregnancies among women aged >40 years are associated with more non-severe complications, more premature births, more congenital malformations and more interventions at birth.

Preferences for place of death if faced with advanced cancer: a population survey in England, Flanders, Germany, Italy, the Netherlands, Portugal and Spain

BACKGROUND

Cancer end-of-life care (EoLC) policies assume people want to die at home. We aimed to examine variations in preferences for place of death cross-nationally.

METHODS

A telephone survey of a random sample of individuals aged ≥16 in England, Flanders, Germany, Italy, the Netherlands, Portugal and Spain. We determined where people would prefer to die if they had a serious illness such as advanced cancer, facilitating circumstances, personal values and experiences of illness, death and dying.

RESULTS

Of 9344 participants, between 51% (95% CI: 48% to 54%) in Portugal and 84% (95% CI: 82% to 86%) in the Netherlands would prefer to die at home. Cross-national analysis found there to be an influence of circumstances and values but not of experiences of illness, death and dying. Four factors were associated with a preference for home death in more than one country: younger age up to 70+ (Germany, the Netherlands, Portugal, Spain), increased importance of dying in the preferred place (England, Germany, Portugal, Spain), prioritizing keeping a positive attitude (Germany, Spain) and wanting to involve family in decisions if incapable (Flanders, Portugal).

CONCLUSIONS

At least two-thirds of people prefer a home death in all but one country studied. The strong association with personal values suggests keeping home care at the heart of cancer EoLC.

Rotavirus gene structure and function

Knowledge of the structure and function of the genes and proteins of the rotaviruses has expanded rapidly. Information obtained in the last 5 years has revealed unexpected and unique molecular properties of rotavirus proteins of general interest to virologists, biochemists, and cell biologists. Rotaviruses share some features of replication with reoviruses, yet antigenic and molecular properties of the outer capsid proteins, VP4 (a protein whose cleavage is required for infectivity, possibly by mediating fusion with the cell membrane) and VP7 (a glycoprotein), show more similarities with those of other viruses such as the orthomyxoviruses, paramyxoviruses, and alphaviruses. Rotavirus morphogenesis is a unique process, during which immature subviral particles bud through the membrane of the endoplasmic reticulum (ER). During this process, transiently enveloped particles form, the outer capsid proteins are assembled onto particles, and mature particles accumulate in the lumen of the ER. Two ER-specific viral glycoproteins are involved in virus maturation, and these glycoproteins have been shown to be useful models for studying protein targeting and retention in the ER and for studying mechanisms of virus budding. New ideas and approaches to understanding how each gene functions to replicate and assemble the segmented viral genome have emerged from knowledge of the primary structure of rotavirus genes and their proteins and from knowledge of the properties of domains on individual proteins. Localization of type-specific and cross-reactive neutralizing epitopes on the outer capsid proteins is becoming increasingly useful in dissecting the protective immune response, including evaluation of vaccine trials, with the practical possibility of enhancing the production of new, more effective vaccines. Finally, future analyses with recently characterized immunologic and gene probes and new animal models can be expected to provide a basic understanding of what regulates the primary interactions of these viruses with the gastrointestinal tract and the subsequent responses of infected hosts.

Evidence-based guideline: Treatment of painful diabetic neuropathy: report of the American Academy of Neurology, the American Association of Neuromuscular and Electrodiagnostic Medicine, and the American Academy of Physical Medicine and Rehabilitation

OBJECTIVE

To develop a scientifically sound and clinically relevant evidence-based guideline for the treatment of painful diabetic neuropathy (PDN).

METHODS

We performed a systematic review of the literature from 1960 to August 2008 and classified the studies according to the American Academy of Neurology classification of evidence scheme for a therapeutic article, and recommendations were linked to the strength of the evidence. The basic question asked was: "What is the efficacy of a given treatment (pharmacologic: anticonvulsants, antidepressants, opioids, others; and nonpharmacologic: electrical stimulation, magnetic field treatment, low-intensity laser treatment, Reiki massage, others) to reduce pain and improve physical function and quality of life (QOL) in patients with PDN?"

RESULTS AND RECOMMENDATIONS

Pregabalin is established as effective and should be offered for relief of PDN (Level A). Venlafaxine, duloxetine, amitriptyline, gabapentin, valproate, opioids (morphine sulfate, tramadol, and oxycodone controlled-release), and capsaicin are probably effective and should be considered for treatment of PDN (Level B). Other treatments have less robust evidence or the evidence is negative. Effective treatments for PDN are available, but many have side effects that limit their usefulness, and few studies have sufficient information on treatment effects on function and QOL.

Factors influencing the success of in vitro fertilization for alleviating human infertility

The program for in vitro fertilization at Bourn Hall began in October 1980. Various types of infertility have been treated during this time using the natural menstrual cycle or stimulation of follicular growth with antiestrogens and gonadotrophins. Follicular growth and maturation are assayed by urinary estrogens and LH, monitored regularly during the later follicular stage. Many patients had an endogenous LH surge; others needed an injection of HCG to induce ovulation. All oocytes were recovered by laparoscopy. Wide variations occurred in the time interval between the start of the LH surge and oocyte recovery and between oocyte recovery and insemination. Embryos taken between the one- and the eight-cell stage were replaced into their mother, no standard procedure being adopted for all patients. The results of all treatments including patient's responses during the follicular and luteal phases, oocyte recovery, fertilization, cleavage, replacement, implantation, abortion, and birth and the effect of factors such as replacing two or more embryos, maternal age, and previous obstetric history are described in detail. The incidence of implantation after embryo replacement improved from 16.5% initially to 30% currently. More than 118 babies have been born, and many pregnancies are continuing.

Trial of Sodium Phenylbutyrate-Taurursodiol for Amyotrophic Lateral Sclerosis

BACKGROUND

Sodium phenylbutyrate and taurursodiol have been found to reduce neuronal death in experimental models. The efficacy and safety of a combination of the two compounds in persons with amyotrophic lateral sclerosis (ALS) are not known.

METHODS

In this multicenter, randomized, double-blind trial, we enrolled participants with definite ALS who had had an onset of symptoms within the previous 18 months. Participants were randomly assigned in a 2:1 ratio to receive sodium phenylbutyrate-taurursodiol (3 g of sodium phenylbutyrate and 1 g of taurursodiol, administered once a day for 3 weeks and then twice a day) or placebo. The primary outcome was the rate of decline in the total score on the Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised (ALSFRS-R; range, 0 to 48, with higher scores indicating better function) through 24 weeks. Secondary outcomes were the rates of decline in isometric muscle strength, plasma phosphorylated axonal neurofilament H subunit levels, and the slow vital capacity; the time to death, tracheostomy, or permanent ventilation; and the time to death, tracheostomy, permanent ventilation, or hospitalization.

RESULTS

A total of 177 persons with ALS were screened for eligibility, and 137 were randomly assigned to receive sodium phenylbutyrate-taurursodiol (89 participants) or placebo (48 participants). In a modified intention-to-treat analysis, the mean rate of change in the ALSFRS-R score was -1.24 points per month with the active drug and -1.66 points per month with placebo (difference, 0.42 points per month; 95% confidence interval, 0.03 to 0.81; P = 0.03). Secondary outcomes did not differ significantly between the two groups. Adverse events with the active drug were mainly gastrointestinal.

CONCLUSIONS

Sodium phenylbutyrate-taurursodiol resulted in slower functional decline than placebo as measured by the ALSFRS-R score over a period of 24 weeks. Secondary outcomes were not significantly different between the two groups. Longer and larger trials are necessary to evaluate the efficacy and safety of sodium phenylbutyrate-taurursodiol in persons with ALS. (Funded by Amylyx Pharmaceuticals and others; CENTAUR ClinicalTrials.gov number, NCT03127514.).

An immunodominant epitope of the human immunodeficiency virus envelope glycoprotein gp160 recognized by class I major histocompatibility complex molecule-restricted murine cytotoxic T lymphocytes

Because cytotoxic T lymphocytes (CTL) may be important for preventing direct cell-to-cell transmission of human immunodeficiency virus (HIV), the agent responsible for acquired immunodeficiency syndrome, we have begun to investigate the epitope specificity and immune response (Ir) gene control of anti-HIV CTL responses in experimental animals. Mice were infected with a recombinant vaccinia virus expressing the HIV gp160 envelope gene, and the primed lymphocytes were restimulated in vitro with a transfected histocompatible cell line expressing the same gene. Our results show that H-2d mice are CTL high responders and H-2k mice are low responders to the HIV gp160 envelope protein under these conditions. Moreover, the H-2d mice respond predominantly to a single immunodominant site represented by a 15-residue synthetic peptide conforming to the amphipathic alpha-helix model of T-cell epitopes and seen by CD4- CD8+ CTL in association with the Dd class I major histocompatibility complex (MHC) molecules. The facts that CTL responses were detected in the context of only one of four class I MHC molecules tested and that the response was limited predominantly to a single epitope indicate that the CTL repertoire elicited by the HIV envelope protein in association with murine class I MHC molecules may be very limited. In addition, this epitope occurs in a highly variable segment of the envelope protein. This puts constraints on the use of a single peptide sequence from this region in a vaccine, as such a vaccine would have to be polyvalent. Nevertheless, this same variability suggests that this region may be under selective pressure from human CTL, and therefore that this site may be immunodominant in humans as well as mice and so of clinical importance in vaccine development.

Implantation enhancement by selective assisted hatching using zona drilling of human embryos with poor prognosis

Assisted hatching by zona drilling using acidic Tyrode's solution was performed during three randomized trials in 330 in-vitro fertilization patients. The trials were designed in order to study the overall effect of the procedure and whether characteristic patient [i.e. maternal age and basal levels of follicle stimulating hormone (FSH)] and embryonic features (i.e. zona pellucida thickness) are important for the decision to perform assisted hatching routinely. Couples (n = 137; Trial 1) in whom the female partner had normal basal FSH levels were randomized in a control group (without micromanipulation) and a zona drilling group (all embryos micromanipulated). The incidence of implantation (67/239; 28%) of zona-drilled embryos compared favourably with that of control embryos (49/229; 21%), but the difference was not significant. Retrospective analysis revealed that those embryos whose zonae were thicker than 15 microns were rescued. In order to test the validity of this finding, selective assisted hatching was performed on embryos with a poor prognosis in 163 other patients (Trial 2). The couples were randomized into a control group and a group in which embryos were selectively zona-drilled, based on zona thickness and other embryonic features. The rate of embryonic implantation in the selectively zona-drilled group was 25% (70/278), significantly (P less than 0.05) higher than that of control embryos (51/285; 18%). Although it was demonstrated retrospectively and prospectively that assisted hatching by zona drilling is effective in embryos with thick zonae (greater than or equal to 15 microns), patients whose embryos have thin (less than 13 microns) zonae may be jeopardized by the procedure.(ABSTRACT TRUNCATED AT 250 WORDS)

Positive outcome after preimplantation diagnosis of aneuploidy in human embryos

usromosomal abnormalities are responsible for a great deal of embryo wastage, which is reflected, at least partially, in decreased implantation and increased miscarriage in older women. To address this problem the transfer of only chromosomally normal embryos previously selected by preimplantation genetic diagnosis (PGD) has been proposed. We designed a multi-centre in-vitro fertilization (IVF) study to compare controls and a test group that underwent embryo biopsy and PGD for aneuploidy. Patients were matched retrospectively, but blindly, for average maternal age, number of previous IVF cycles, duration of stimulation, oestradiol concentrations on day +1, and average mature follicles. All these parameters were similar in test and control groups. Only embryos classified as normal for those chromosomes were transferred after PGD. The results showed that the rates of fetal heart beat (FHB)/embryo transferred between the control and test groups were similar. However, spontaneous abortions, measured as FHB aborted/FHB detected, decreased after PGD (P < 0.05), and ongoing pregnancies and delivered babies increased (P < 0.05) in the PGD group of patients. Two conclusions were obtained: (i) PGD of aneuploidy reduced embryo loss after implantation; (ii) implantation rates were not significantly improved, but the proportion of ongoing and delivered babies was increased.

Induction of anti-phospholipid syndrome in naive mice with mouse lupus monoclonal and human polyclonal anti-cardiolipin antibodies

The primary anti-phospholipid syndrome is characterized by recurrent venous and arterial thromboembolic phenomena, recurrent fetal loss, thrombocytopenia, and serological evidence of anti-cardiolipin (aCL) antibodies or/and the presence of lupus anticoagulant (prolonged activated partial thromboplastin time). The exact role of aCL antibodies in pathogenesis is not clear and the mechanism by which the antibodies may induce the various manifestations is unknown. In the current study we evaluated the effect of passive transfer of aCL antibodies (to the tail vein of naive mice) on fecundity, fetal loss (fetal resorption), and the weight of embryos and placentae. Two types of aCL antibodies were employed: (i) mouse monoclonal aCL antibodies derived from a BALB/c mouse in which experimental systemic lupus erythematosus was induced by a pathogenic idiotype (idiotype 16/6) of anti-DNA antibodies and (ii) polyclonal IgG and IgM aCL antibodies derived from serum of a patient with primary anti-phospholipid syndrome. After infusion of either antibody (10 micrograms per mouse) we could demonstrate lower fecundity rate, increased resorption index of embryos (equivalent to recurrent fetal loss), lower number of embryos per pregnancy, and lower mean weights of embryos and placentae in comparison to mice infused with appropriate control immunoglobulins. We conclude that the aCL antibodies may have direct effects on fecundity and on the outcome of pregnancy.

INTERSEPT: an international, multicenter, placebo-controlled trial of monoclonal antibody to human tumor necrosis factor-alpha in patients with sepsis. International Sepsis Trial Study Group

OBJECTIVE

To determine the safety and efficacy of BAY x 1351, a murine monoclonal antibody to recombinant human tumor necrosis factor (TNF)-alpha, in patients with sepsis.

DESIGN

An international, multicenter, prospective, placebo-controlled trial in patients with sepsis, stratified into shock/nonshock groups.

SETTING

Forty acute clinical care facilities in 14 countries.

PATIENTS

Of the 564 patients enrolled in the study, 553 patients received study drug or placebo.

INTERVENTIONS

Patients received 15 mg/kg or 3 mg/kg of BAY x 1351, or placebo, as a single intravenous infusion.

MEASUREMENTS AND MAIN RESULTS

The patients were well matched for severity of illness and for risk factors known to influence the outcome of sepsis. There was no difference in 28-day mortality rates between groups (placebo group 66 [39.5%] of 167;3 mg/kg group 57 [31.5%] of 181; 15 mg/kg group 87 [42.4%] of 205). Approximately 9 months after this study had begun, an interim safety examination of NORASEPT, a North American Sepsis Trial using the same monoclonal antibody, indicated that there was no benefit to patients in the nonshock group and further enrollment of these nonshock septic patients into INTERSEPT was stopped. The analysis therefore focused on the 420 patients in shock. The primary efficacy variable was the 28-day, all-cause mortality rate: placebo group 57 (42.9%) of 133; 3-mg/kg group 51 (36.7%) of 139; and 15-mg/kg group 66 (44.6%) of 148 (not significant). Two secondary efficacy variables were identified prospectively: shock reversal and frequency of organ failures. Life-table analysis showed that in patients who survived 28 days, there was a more rapid reversal of shock in both treatment groups compared with placebo (15-mg/kg group vs. placebo group log-rank statistic p = .007, 3-mg/kg group vs. placebo group p = .01). Similarly, in patients surviving 28 days, there was a significant delay in the time to the onset of first organ failure (log rank 15 mg/kg vs. placebo p = .03, 3 mg/kg vs. placebo p = .07), and more patients in the placebo group developed at least one organ failure: 15-mg/kg group 33 (40.2%) of 82; 3-mg/kg group 39 (44.3%) of 88; and placebo group 45 (59.2%) of 76 (15 mg/kg vs. placebo p = .03, 3 mg/kg vs. placebo p = .06). No significant adverse events were associated with the monoclonal antibody treatment.

CONCLUSIONS

INTERSEPT provides additional clinical data implicating TNF-alpha as an integral mediator of septic shock. The study suggested a possible role for anti-TNF antibody as adjunctive therapy, but this possibility requires confirmation by another clinical trial.

The clinician's illusion

There are several diseases, including schizophrenia, alcoholism, and opiate addiction, for which the long-term prognosis is subject to disagreement between clinicians and researchers and also among researchers. Part of this disagreement may be attributable to a difference in the populations they sample. The clinician samples the population currently suffering from the disease (a "prevalence" or census sample), while research samples tend to more nearly represent the population ever contracting the disease (an "incidence" sample). The clinician's sample is biased toward cases of long duration, since the probability that a case will appear in a prevalence sample is proportional to its duration, hence "the clinician's illusion." The statistical mechanism of this bias is illustrated and its consequences detailed. Other sources of sampling bias in clinical and research samples are briefly described and partial remedies are suggested.