Clozapine, olanzapine, risperidone, and haloperidol in the treatment of patients with chronic schizophrenia and schizoaffective disorder

OBJECTIVE: The authors compared the efficacy and safety of three atypical antipsychotics (clozapine, olanzapine, and risperidone) with one another and with haloperidol in the treatment of patients with chronic schizophrenia or schizoaffective disorder. METHOD: In a double-blind trial, 157 inpatients with a history of suboptimal treatment response were randomly assigned to treatment with clozapine, olanzapine, risperidone, or haloperidol for 14 weeks (an 8-week escalation and fixed-dose period followed by a 6-week variable-dose period). RESULTS: Clozapine, risperidone, and olanzapine (but not haloperidol) resulted in statistically significant improvements in total score on the Positive and Negative Syndrome Scale. Improvements seen in total and negative symptom scores with clozapine and olanzapine were superior to haloperidol. The atypical drugs, particularly olanzapine and clozapine, were associated with weight gain. CONCLUSIONS: The effects of atypical antipsychotics in this population were statistically significant but clinically modest. The overall pattern of results suggests that clozapine and olanzapine have similar general antipsychotic efficacy and that risperidone may be somewhat less effective. Clozapine was the most effective treatment for negative symptoms. However, the differences among treatments were small.     

Pharmacotherapy and health care costs among patients with schizophrenia and newly diagnosed diabetes

OBJECTIVE: This study investigated how antipsychotic pharmacotherapy and health care costs change after diabetes mellitus is newly diagnosed among patients with schizophrenia. METHODS: Administrative data from the Department of Veterans Affairs were retrospectively reviewed to examine patients with schizophrenia who did not have any history of diabetes and for whom a consistent regimen of antipsychotic monotherapy was prescribed for any three-month period between June 1999 and September 2000. Data for these patients were reviewed through September 2001. Patients who were given a new diagnosis of diabetes were identified, along with a matched comparison group of patients who were not given a diagnosis of diabetes. Medication changes and costs were compared between patients with diabetes and those without and between patients who were taking second-generation antipsychotics and those who were taking first-generation antipsychotics. RESULTS: Of the 56,849 patients who fit the criteria for the study, 4,132 (7.3 percent) were subsequently given a diagnosis of diabetes (7.4 percent were taking second-generation antipsychotics and 7.1 percent were taking first-generation antipsychotics). Differences in the proportions of patients with and without diabetes who switched or discontinued antipsychotics were small and were statistically significant only for patients who were taking risperidone before the diabetes diagnosis date. The average marginal cost of treating a patient with diabetes was 3,104 US dollars over an average follow-up of 15.7 months, or 6.59 US dollars per day. Because the attributable risks of diabetes with second-generation antipsychotics averaged .875 percent, the average additional daily cost per patient that was attributable to each second-generation medication was small, ranging from .003 US dollars for risperidone to .134 US dollars for clozapine. CONCLUSIONS: Surprisingly, a new diagnosis of diabetes did not result in substantial antipsychotic medication changes, even among patients who were taking clozapine or olanzapine. Even though the costs of treating patients with newly diagnosed diabetes were substantial, the increased costs attributable to second-generation antipsychotics were small.     

Equivalent occupancy of dopamine D1 and D2 receptors with clozapine: differentiation from other atypical antipsychotics

OBJECTIVE: Clozapine, the prototype of atypical antipsychotics, remains unique in its efficacy in the treatment of refractory schizophrenia. Its affinity for dopamine D(4) receptors, serotonin 5-HT(2A) receptor antagonism, effects on the noradrenergic system, and its relatively moderate occupancy of D(2) receptors are unlikely to be the critical mechanism underlying its efficacy. In an attempt to elucidate the molecular/synaptic mechanism underlying clozapine's distinctiveness in refractory schizophrenia, the authors studied the in vivo D(1) and D(2) receptor profile of clozapine compared with other atypical antipsychotics. METHOD: Positron emission tomography with the radioligands [(11)C]SCH23390 and [(11)C]raclopride was used to investigate D(1) and D(2) receptor occupancy in vivo in 25 schizophrenia patients receiving atypical antipsychotic treatment with clozapine, olanzapine, quetiapine, or risperidone. RESULTS: Mean striatal D(1) occupancies ranged from 55% with clozapine to 12% with quetiapine (rank order: clozapine > olanzapine > risperidone > quetiapine). The striatal D(2) occupancy ranged from 81% with risperidone to 30% with quetiapine (rank order: risperidone > olanzapine > clozapine > quetiapine). The ratio of striatal D(1)/D(2) occupancy was significantly higher for clozapine (0.88) relative to olanzapine (0.54), quetiapine (0.41), or risperidone (0.31). CONCLUSIONS: Among the atypical antipsychotics, clozapine appears to have a simultaneous and equivalent occupancy of dopamine D(1) and D(2) receptors. Whether its effect on D(1) receptors represents agonism or antagonism is not yet clear, as this issue is still unresolved in the preclinical arena. This distinctive effect on D(1)/D(2) receptors may be responsible for clozapine's unique effectiveness in patients with schizophrenia refractory to other typical and atypical antipsychotics.     

Incidence of newly diagnosed diabetes attributable to atypical antipsychotic medications

OBJECTIVE: The purpose of the study was to determine the proportion of patients with schizophrenia with a stable regimen of antipsychotic monotherapy who developed diabetes or were hospitalized for ketoacidosis. METHOD: Patients with schizophrenia for whom a stable regimen of antipsychotic monotherapy was consistently prescribed during any 3-month period between June 1999 and September 2000 and who had no diabetes were followed through September 2001 by using administrative data from the Department of Veterans Affairs. Cox proportional hazards models were developed to identify the characteristics associated with newly diagnosed diabetes and ketoacidosis. RESULTS: Of the 56,849 patients identified, 4,132 (7.3%) developed diabetes and 88 (0.2%) were hospitalized for ketoacidosis. Diabetes risk was highest for clozapine (hazard ratio=1.57) and olanzapine (hazard ratio=1.15); the diabetes risks for quetiapine (hazard ratio=1.20) and risperidone (hazard ratio=1.01) were not significantly different from that for conventional antipsychotics. The attributable risks of diabetes mellitus associated with atypical antipsychotics were small, ranging from 0.05% (risperidone) to 2.03% (clozapine). CONCLUSIONS: Although clozapine and olanzapine have greater diabetes risk, the attributable risk of diabetes mellitus with atypical antipsychotics is small.     

Effects of atypical antipsychotics on the syndromal profile in treatment-resistant schizophrenia

BACKGROUND: There has been considerable support for the observation that atypical antipsychotics have a broader range of therapeutic effects than traditional antipsychotics. We are exploring whether this expanded clinical efficacy can also be seen in patients with treatment-resistant schizophrenia. METHOD: The subjects were 157 treatment-resistant inpatients diagnosed with DSM-IV schizophrenia or schizoaffective disorder. They were randomly assigned to treatment with clozapine, olanzapine, risperidone, or haloperidol in a 14-week double-blind trial and rated with a standard measure of clinical antipsychotic efficacy (Positive and Negative Syndrome Scale [PANSS]). Factor analysis at baseline and endpoint together with changes in 5 PANSS-derived factors were examined. Data were gathered from June 1996 to December 1999. RESULTS: The underlying PANSS factor structure, as indicated by the factor loadings, was essentially identical at baseline and endpoint. At baseline, the excitement factor was followed by the positive, negative, cognitive, and depression/anxiety factors, explaining 49.4% of the total variance. At endpoint, the positive factor was followed by the negative, excitement, cognitive, and depression/anxiety factors, explaining 55.5% of the total variance. The endpoint data indicated statistically significant (p <.05) improvements over time on the positive factor for all 3 atypicals, but not for haloperidol. The negative factor showed significant improvement for clozapine and olanzapine, with significant worsening for haloperidol. Clozapine, olanzapine, and risperidone were superior to haloperidol on the negative factor, while clozapine was also superior to risperidone. The cognitive factor showed significant improvement for all atypicals, as did the depression/anxiety factor. Only clozapine showed improvement on the excitement factor and was superior to both haloperidol and risperidone. CONCLUSIONS: Treatment with atypical antipsychotics did not substantially change the underlying PANSS 5-factor structure. However, antipsychotic treatment with all 3 atypical medications was associated with significant improvements on 3 of 5 syndromal domains (positive, cognitive, and depression/anxiety) of schizophrenia. Clozapine and olanzapine also showed improvement on the negative factor. Only clozapine was associated with improvement on the excitement domain. This finding confirms that atypicals are associated with improvement of an expanded spectrum of symptoms in treatment-resistant patients.     

Serum leptin and triglyceride levels in patients on treatment with atypical antipsychotics

BACKGROUND: Weight gain is a common adverse effect associated with the use of most antipsychotic drugs. Leptin has been reported to be associated with antipsychotic-induced weight gain. Previous studies have demonstrated a relationship between the atypical antipsychotics clozapine and olanzapine and serum leptin levels. We planned to comparatively investigate the effects of the atypical antipsychotics quetiapine, olanzapine, risperidone, and clozapine on leptin and triglyceride levels and weight gain. METHOD: The study population comprised 56 patients with DSM-IV schizophrenia, who were divided into 4 treatment groups: quetiapine (N = 14), olanzapine (N = 14), risperidone (N = 14), or clozapine (N = 14) monotherapy, and a control group of 11 patients receiving no psychopharmacologic treatment. The patients were evaluated at baseline and at the sixth week according to the Positive and Negative Syndrome Scale (PANSS), body mass index (BMI), weight, and fasting serum leptin and triglyceride levels. Data were gathered in 2001 and 2002. RESULTS: Olanzapine and clozapine caused a marked increase in weight and serum triglyceride and leptin levels, though increases in these variables were modest in the patients receiving quetiapine and minimal in those receiving risperidone. There were positive correlations between serum leptin levels and BMI and triglyceride levels. Clinical efficacy, as indicated by decrease in total PANSS scores, was associated with leptin levels in all atypical antipsychotic groups. CONCLUSION: Our results suggest that leptin may be associated with olanzapine- and clozapine-induced weight gain and that quetiapine appears to have modest influence and risperidone appears to have minimal influence on leptin and triglyceride levels and weight gain compared with olanzapine and clozapine.     

Novel antipsychotics: comparison of weight gain liabilities.

BACKGROUND: We performed a retrospective analysis of 122 clinical records of 92 male patients with DSM-III-R schizophrenia to examine the relative weight gain liabilities of clozapine, risperidone, olanzapine, and sertindole compared with haloperidol. We hypothesized that the unique pharmacodynamic profiles of these agents would contribute to different amounts and patterns of weight gain. METHOD: Data were analyzed to determine differences in weight gain during treatment among patients receiving 5 different drug treatments (clozapine [N = 20], olanzapine [N = 13], risperidone [N = 38], haloperidol [N = 43], and sertindole [N = 8]). Measures of maximal weight gain, final weight, and duration to maximal weight gain were calculated. RESULTS: Repeated measures analyses of variance controlling for age, treatment duration, and initial weight revealed statistically significant differences between groups on all 3 measures. Clozapine and olanzapine had the greatest maximal weight gain liability (F = 4.13, df = 4,23; p = .01). Weight gain with clozapine, but not olanzapine or risperidone, appears to persist (as reflected by final weight) despite behavioral interventions (e.g., nutritional consultation, suggested exercise regimen; F = 5.69, df = 4,23; p = .003). Clozapine- and olanzapine-treated subjects appeared to gain weight over a prolonged period of time, whereas risperidone-and sertindole-treated subjects had a more limited period of weight gain (F = 2.95, df = 4,25; p = .04). CONCLUSION: Clozapine and olanzapine caused the most weight gain, risperidone was intermediate, and sertindole had less associated weight gain than haloperidol. The relative receptor affinities of the novel antipsychotics for histamine H1 appear to be the most robust correlate of these clinical findings.     

Elevation of prolactin levels by atypical antipsychotics.

OBJECTIVE: Atypical antipsychotics are thought not to elevate prolactin levels. The authors examined data suggesting that atypical antipsychotics do elevate prolactin levels but more transiently than typical antipsychotics. METHOD: Prolactin levels in 18 male patients with schizophrenia who were receiving atypical antipsychotics were monitored over the 24-hour period following administration of their daily oral dose of risperidone, olanzapine, or clozapine. RESULTS: The baseline prolactin levels in patients receiving risperidone (mean=27 ng/ml, SD=14) were abnormally high, but baseline prolactin levels in patients receiving olanzapine (mean=9 ng/ml, SD=5) and clozapine (mean=9 ng/ml, SD=5) were not high. All three atypical antipsychotics caused a doubling of prolactin levels over baseline levels 6 hours after medication administration. CONCLUSIONS: These data suggest that these atypical antipsychotics raise prolactin levels, although the increases with olanzapine did not reach statistical significance. This suggests that the differences in the effects on prolactin levels of atypical and typical antipsychotics are not categorical but lie in the degree and duration of dose-induced prolactin elevation, attributable to the differential binding properties of each drug on pituitary dopamine D(2) receptors.     

Effectiveness of clozapine versus olanzapine, quetiapine, and risperidone in patients with chronic schizophrenia who did not respond to prior atypical antipsychotic treatment

OBJECTIVE: When a schizophrenia patient has an inadequate response to treatment with an antipsychotic drug, it is unclear what other antipsychotic to switch to and when to use clozapine. In this study, the authors compared switching to clozapine with switching to another atypical antipsychotic in patients who had discontinued treatment with a newer atypical antipsychotic in the context of the Clinical Antipsychotic Trials for Interventions Effectiveness (CATIE) investigation. METHOD: Ninety-nine patients who discontinued treatment with olanzapine, quetiapine, risperidone, or ziprasidone in phase 1 or 1B of the trials, primarily because of inadequate efficacy, were randomly assigned to open-label treatment with clozapine (N=49) or blinded treatment with another newer atypical antipsychotic not previously received in the trial (olanzapine [N=19], quetiapine [N=15], or risperidone [N=16]). RESULTS: Time until treatment discontinuation for any reason was significantly longer for clozapine (median=10.5 months) than for quetiapine (median=3.3), or risperidone (median=2.8), but not for olanzapine (median=2.7). Time to discontinuation because of inadequate therapeutic effect was significantly longer for clozapine than for olanzapine, quetiapine, or risperidone. At 3-month assessments, Positive and Negative Syndrome Scale total scores had decreased more in patients treated with clozapine than in patients treated with quetiapine or risperidone but not olanzapine. One patient treated with clozapine developed agranulocytosis, and another developed eosinophilia; both required treatment discontinuation. CONCLUSIONS: For these patients with schizophrenia who prospectively failed to improve with an atypical antipsychotic, clozapine was more effective than switching to another newer atypical antipsychotic. Safety monitoring is necessary to detect and manage clozapine's serious side effects.     

Effectiveness of olanzapine, quetiapine, risperidone, and ziprasidone in patients with chronic schizophrenia following discontinuation of a previous atypical antipsychotic

BACKGROUND: In the treatment of schizophrenia, changing antipsychotics is common when one treatment is suboptimally effective, but the relative effectiveness of drugs used in this strategy is unknown. This randomized, double-blind study compared olanzapine, quetiapine, risperidone, and ziprasidone in patients who had just discontinued a different atypical antipsychotic. METHOD: Subjects with schizophrenia (N=444) who had discontinued the atypical antipsychotic randomly assigned during phase 1 of the Clinical Antipsychotic Trials of Intervention Effectiveness (CATIE) investigation were randomly reassigned to double-blind treatment with a different antipsychotic (olanzapine, 7.5-30 mg/day [N=66]; quetiapine, 200-800 mg/day [N=63]; risperidone, 1.5-6.0 mg/day [N=69]; or ziprasidone, 40-160 mg/day [N=135]). The primary aim was to determine if there were differences between these four treatments in effectiveness measured by time until discontinuation for any reason. RESULTS: The time to treatment discontinuation was longer for patients treated with risperidone (median: 7.0 months) and olanzapine (6.3 months) than with quetiapine (4.0 months) and ziprasidone (2.8 months). Among patients who discontinued their previous antipsychotic because of inefficacy (N=184), olanzapine was more effective than quetiapine and ziprasidone, and risperidone was more effective than quetiapine. There were no significant differences between antipsychotics among those who discontinued their previous treatment because of intolerability (N=168). CONCLUSIONS: Among this group of patients with chronic schizophrenia who had just discontinued treatment with an atypical antipsychotic, risperidone and olanzapine were more effective than quetiapine and ziprasidone as reflected by longer time until discontinuation for any reason.     

Comparison of cost, dosage and clinical preference for risperidone and olanzapine

BACKGROUND: Because risperidone and olanzapine have similar efficacy and tolerability in the treatment of schizophrenia, costs, physician experience, and preference become relevant considerations in making treatment decisions. The purpose of this paper is to compare daily treatment costs of risperidone and olanzapine, and to examine psychiatrists' clinical preferences. METHOD: Dosage information was obtained from a national Ministry of Health registry and a national survey of psychiatrists. In addition, psychiatrists' clinical preference of antipsychotic medication and dosage for patient subtypes were examined by the national survey. RESULTS: Data from the registry and national survey estimated the mean daily dose of risperidone to be one-third that of olanzapine, irrespective of patient subtype. Taking into account drug costs and dosage requirements, the average daily retail price was US $6.85 for risperidone and US $13.60 for olanzapine. Psychiatrists preferred risperidone for first-episode psychosis and elderly psychosis, and olanzapine for patients sensitive to EPS. They rated the drugs equally effective on positive and negative symptoms, for chronic patients, for treatment-refractory patients and relapse prevention. CONCLUSIONS: Risperidone has a substantial cost advantage over olanzapine, and was preferred by psychiatrists for more indications.     

Managing treatment-resistant schizophrenia: evidence from randomized clinical trials

Clozapine was the first antipsychotic medication to be approved for the indication of treatment-refractory schizophrenia. This followed rigorous testing in patients who retrospectively and prospectively failed treatment trials of relatively high doses of conventional antipsychotics. In the past decade, other atypical antipsychotics have been approved, but they have not been designated specifically for patients with a history of prior poor treatment response. Better tolerated than clozapine, these new agents have been used with varying success in patients who would have otherwise received clozapine. Up until very recently there has not been a head-to-head controlled clinical trial comparing the two most commonly used atypical antipsychotics, risperidone and olanzapine, with clozapine in patients considered to have a suboptimal response to typical antipsychotics. This review summarizes the current advances made in the pharmacological management of these patients by examining recently published randomized controlled clinical trials that have measured psychopathology and cognition.     

Comparison of risperidone and olanzapine as used under "real-world" conditions in a state psychiatric hospital

As a follow-up to our previous study of clozapine, medical records of a state psychiatric hospital were reviewed for patients who were prescribed an atypical antipsychotic. From that sample, demographic and clinical data were obtained for individuals with an initial score of 35 or greater on the Brief Psychiatric Rating Scale (BPRS), and at least two additional successive monthly BPRS ratings. A total of 100 patients met the criteria. Most received either olanzapine (46%) or risperidone (36%), with few administered quetiapine (11%) or clozapine (7%). Most also received adjunctive medications, including conventional antipsychotics, anticonvulsants/mood stabilizers, antidepressants, and antiparkinsonian agents. The number of patients whose BPRS total scores decreased by 20% or more from baseline was significantly greater for those who received olanzapine than those who received risperidone. However, there was no difference between the two antipsychotics in the number of patients who maintained that degree of improvement, in the average latency to achieve that decrease (1.67 and 1.47 months, respectively), or the average length of stay (LOS; 332 and 376 days, respectively). These results indicate a modest therapeutic advantage of olanzapine compared to risperidone, and a substantial degree of polypharmacy in the use of atypical antipsychotics. This uncontrolled "real-world" evaluation supports data from controlled clinical trials, showing that either risperidone or olanzapine would be a reasonable first choice in patients with treatment-resistant schizophrenia, with the decision based on the least adverse side effect profile and economic constraints. When compared to our previous clozapine study, we confirm a slight advantage for the effectiveness of clozapine in the treatment of this refractory population.     

Atypical antipsychotics in ordinary clinical practice: a pharmaco-epidemiologic survey in a south London service.

The introduction of atypical antipsychotics has triggered a reevaluation of treatment strategies in schizophrenia. Although research findings inform about the efficacy and safety of drugs, it is the decisions made daily in ordinary practice that affect the vast majority of patients. The aim of this paper was to examine the use of atypical antipsychotics in clinical care, by means of a survey of prescribing practices for clozapine, risperidone, olanzapine, sertindole and quetiapine for all patients treated for DSM-IV schizophrenia within a psychiatric service. Seventy-five (26%) of the 286 patients identified were on atypical antipsychotics. Patients on clozapine were mostly male, had more than 15 years of contact with psychiatric services and were poor responders. Patients prescribed other atypicals had responded to prior treatment with typical neuroleptics and had less than five years of contact with psychiatric services. Gender distribution was equal in this group. The use of research criteria for treatment resistance identified only one patient, as prolonged use of high-dose neuroleptics was uncommon. The new atypical antipsychotics appear to be replacing older neuroleptics as the first-line treatment of schizophrenia while clozapine is mostly reserved for poor responders. Clinicians' definition of treatment resistance was variable and below the threshold used in research.     

Effects of atypical and typical antipsychotic treatments on sexual function in patients with schizophrenia: 12-month results from the Intercontinental Schizophrenia Outpatient Health Outcomes (IC-SOHO) study.

PURPOSE: Sexual dysfunction in patients with schizophrenia can reduce quality of life and treatment compliance. This report will compare the effects of selected atypical and typical antipsychotics on sexual function in a large, international population of outpatients with schizophrenia who were treated over 1 year. SUBJECTS AND METHODS: Outpatients with schizophrenia, who initiated or changed antipsychotic treatment, and entered this 3-year, prospective, observational study were classified according to the monotherapy prescribed at baseline: olanzapine (N=2638), risperidone (N=860), quetiapine (N=142) or haloperidol (N=188). RESULTS: Based on patient perception, the odds of experiencing sexual dysfunction during 1 year of therapy was significantly lower for patients treated with olanzapine and quetiapine when compared to patients who received risperidone or haloperidol (all P< or =0.001). Females on olanzapine (14%) or quetiapine (8%) experienced a lower rate of menstrual irregularities, compared to females on risperidone (23%) or haloperidol (29%). Significant discordance was evident between patient reports and psychiatrist perception of sexual dysfunction, with psychiatrists underestimating sexual dysfunction (P< or =0.001). CONCLUSION: These findings indicate clinically relevant differences exist in the sexual side effect profiles of these selected antipsychotics. These factors should be considered when selecting the most appropriate treatment for outpatients with schizophrenia.     

Schizophrenic patients treated with clozapine or olanzapine perform better on theory of mind tasks than those treated with risperidone or typical antipsychotic medications

Theory of mind (ToM), the ability to attribute mental states to others, is associated with medial prefrontal cortical (mPFC) activity and is impaired in schizophrenia. Olanzapine or clozapine but not typical antipsychotics or risperidone preferentially affect c-fos expression in mPFC in animals. We tested the hypothesis that schizophrenic patients treated with different antipsychotics would perform differently on ToM tasks. Groups receiving Typicals (n=23), Clozapine (n=18), Olanzapine (n=20) or Risperidone (n=23) and a Control group of healthy volunteers (n=24) were matched for age, gender, handedness and education. ToM functioning was assessed with picture sequence, second-order belief and faux-pas tests. Schizophrenic groups performed similarly to controls on non-ToM conditions. The Olanzapine and Clozapine groups performed similarly to Controls on ToM tasks. The Typicals and Risperidone groups performed worse than the other groups on ToM tasks. We concluded that ToM performance of schizophrenic patients is influenced by the antipsychotic they are taking. Our results suggest that olanzapine or clozapine but not typicals or risperidone may improve or protect ToM ability.     

Time to rehospitalization in patients with schizophrenia discharged on first generation antipsychotics, non-clozapine second generation antipsychotics, or clozapine

Rehospitalization is an important outcome of drug effectiveness in schizophrenia. In this study, the hypothesis that clozapine and some second generation antipsychotics (SGA) were superior to first generation antipsychotics (FGA) in preventing rehospitalization of patients with schizophrenia discharged from a university hospital in Brazil was tested. A retrospective observational study was conducted designed to evaluate time to rehospitalization of patients with schizophrenia discharged on a regimen of oral FGA, depot FGA, risperidone, olanzapine and amisulpride, other SGA, or clozapine, during a three-year follow-up period. Risk factors associated with rehospitalization were examined. Of the 464 patients with schizophrenia discharged from hospital, 242 met criteria for study entry. Higher rehospitalization rates were observed in patients treated with depot FGA (30%), risperidone (30%) and other SGA groups (28.5%), respectively. Clozapine was significantly associated with lower rehospitalization risk compared with risperidone. The risk of rehospitalization in patients on olanzapine and amisulpride, and oral FGA, was similar to that of patients in use of clozapine. These results however, are limited by the heterogeneity of illness severity across the groups.