Background For many years it has been claimed that observationalstudies find stronger treatment effects than randomized, controlledtrials. We compared the results of observational studies withthose of randomized, controlled trials.
Methods We searched the Abridged Index Medicus and Cochranedata bases to identify observational studies reported between1985 and 1998 that compared two or more treatments or interventionsfor the same condition. We then searched the Medline and Cochranedata bases to identify all the randomized, controlled trialsand observational studies comparing the same treatments forthese conditions. For each treatment, the magnitudes of theeffects in the various observational studies were combined bythe Mantel–Haenszel or weighted analysis-of-variance procedureand then compared with the combined magnitude of the effectsin the randomized, controlled trials that evaluated the sametreatment.
Results There were 136 reports about 19 diverse treatments,such as calcium-channel–blocker therapy for coronary arterydisease, appendectomy, and interventions for subfertility. Inmost cases, the estimates of the treatment effects from observationalstudies and randomized, controlled trials were similar. In only2 of the 19 analyses of treatment effects did the combined magnitudeof the effect in observational studies lie outside the 95 percentconfidence interval for the combined magnitude in the randomized,controlled trials.
Conclusions We found little evidence that estimates of treatmenteffects in observational studies reported after 1984 are eitherconsistently larger than or qualitatively different from thoseobtained in randomized, controlled trials.
Observational studies have several advantages over randomized,controlled trials, including lower cost, greater timeliness,and a broader range of patients.1 Concern about inherent biasin these studies, however, has limited their use in comparingtreatments.2,3 Observational studies are used primarily to identifyrisk factors and prognostic indicators and in situations inwhich randomized, controlled trials would be impossible or unethical.4
The empirical assessment of observational studies rests largelyon a number of influential comparative studies from the 1970sand 1980s.5,6,7,8,9 These studies suggested that observationalstudies inflate positive treatment effects, as compared withrandomized, controlled trials. In one major study, Chalmerset al.6 showed that 56 percent of nonrandomized trials yieldedfavorable treatment effects, as compared with 30 percent ofblinded, randomized, controlled trials. Three other studieshad similar results.7,8,9 According to many experts, these resultsmean that observational studies should not be used for definingevidence-based medical care: "If you find that [a] study wasnot randomized, we'd suggest that you stop reading it and goon to the next article."10
Evaluations of observational studies have primarily includedstudies from the 1960s and 1970s. We evaluated observationalstudies reported between 1985 and 1998, studies which may bemethodologically superior to earlier studies. Possible methodologicimprovements include a more sophisticated choice of data setsand better statistical methods. Newer methods may have eliminatedsome systematic bias.
Methods
Search for Observational Studies
Observational studies were found by systematically searchingMedline and the Cochrane Database of Systematic Reviews forstudies reported from 1985 through 1998. Although Medline isnow indexed for highly sensitive searches for randomized, controlledtrials, "observational studies" is not an indexable conceptin Medline, and there is no search term for observational studies(Wright N, National Library of Medicine: personal communication).Therefore, we used a text-word strategy to search for "observational,""cohort," "retrospective," "cross-sectional," and "nonrandomized."We limited the search to journals in the Abridged Index Medicus,which indexes the 120 most widely read, prestigious clinicaljournals. To restrict the search to studies comparing treatments,we added the Medline tag "comparative study/," defined as acomparison of any two or more concepts from any Medical SubjectHeading category.
This strategy identified 3868 articles. We reviewed the abstractsof these articles and selected only those that met four criteria.First, the study was not experimental — that is, treatmentswere not assigned for purposes of research. Second, the studyassessed the difference between two treatments or between onetreatment and no treatment. Third, the treatments were implementedby physicians. Studies of diet, exercise, lifestyle changes,or nonprescription medication were not included, since the typeof bias in these studies differs from the type of bias in studiesof physician-implemented treatment. Fourth, the study includeda control group.
Search for Related Studies
When an article that met all four criteria was identified, wesearched the entire Medline data base from 1966 to 1998 forall corresponding randomized, controlled trials and observationalstudies — that is, those that compared the same two treatments(or the same treatment and no treatment), used the same outcomemeasure, and used the same inclusion criteria for patients.We included a few studies that did not have the same inclusioncriteria or follow-up times as the observational studies; thesestudies are noted in the Results.
The Cochrane Database was searched by the same strategy usedwith Medline. This data base is a continuously updated seriesof reviews by members of the Cochrane Collaboration, an internationalorganization that collects research information on the effectsof health care interventions.11 Search of the Cochrane Databasefound three additional treatments for which there were bothobservational studies and randomized, controlled trials.
Additional articles were found by searching the reference listsof the reviewed articles and by searching for pseudo-randomizedstudies with the Medical Subject Heading "controlled clinicaltrial." Pseudo-randomized studies are controlled trials thatassign treatments in a nonrandomized way, such as by givingthe same treatment to every other admitted patient.12 The resultsof the pseudo-randomized trials are reported separately fromthe results of both the observational studies and the randomized,controlled trials.
No ideal criteria were available to evaluate the sensitivityof our search strategy. However, the United Kingdom Health TechnologyAssessment Group recently completed a systematic review of 22treatments that were the subject of both randomized, controlledtrials and observational studies.13 For an estimate of the sensitivityof our search strategy, we compared our search results withtheirs.
Statistical Analysis
We compared the magnitudes of the effects of treatment on principaloutcomes in observational studies and randomized, controlledtrials. We used the Cochrane Collaboration's RevMan and MetaViewsoftware (version 3.1) to combine the magnitudes of the effectsof a treatment in corresponding studies that had the same design.For binary outcomes, we used the Mantel–Haenszel methodfor estimating the overall odds ratios. For continuous outcomemeasures, we used a fixed-effects, weighted analysis-of-variancemodel, with the inverse of the variance of the magnitude ofthe effect as the weighting factor. The use of fixed-effectsmodels exaggerates differences between the results of observationalstudies and randomized, controlled trials if the actual treatmenteffects vary among study populations.
Five studies included in our analysis did not report a confidenceinterval for the magnitude of the effect. For three of thesestudies, we estimated the confidence interval from the magnitudeof the effect and the P value. Our procedure was as follows:we transformed the magnitude of the effect into a statisticwith an approximately normal distribution (e.g., the log ofthe odds ratio); we transformed the P value into a normal teststatistic; we used the transformed magnitude of the effect andthe P value to compute the standard error of the transformedmagnitude of the effect; we used this information to createa 95 percent confidence interval for the transformed magnitudeof the effect; and we used this confidence interval to createa confidence interval for the untransformed magnitude of theeffect. Although these confidence intervals may not be identicalto those that could be computed by other means, they shouldprovide a qualitative indication of the degree of precisionwith which the magnitude of the effect was estimated.
Results
We found 19 treatment comparisons that were the subject of atleast one observational study and at least one randomized, controlledtrial. There were 53 observational studies and 83 randomized,controlled trials. Two additional studies of these treatmentswere pseudo-randomized.
Our search identified the studies for 13 of the 22 treatmentcomparisons identified by the United Kingdom Health TechnologyAssessment Group. Among the other nine treatment comparisons,four had not been the subject of an observational study in ajournal listed in the Abridged Index Medicus, two had not beenthe subject of both a randomized, controlled trial and an observationalstudy, one had not been the subject of an observational studyafter 1984, one was not a medical treatment, and one had notbeen the subject of a study indexed under the Medical SubjectHeading "comparative study/."
In the selection of corresponding studies, there may have beendifferences in how some of the treatments were administered(e.g., evaluations by geriatric assessment units) or in howsome of the outcomes were assessed (e.g., the incidence of infection,recurrent dysphagia, or retinopathy). Both follow-up times andinclusion criteria were identical for 15 of the 19 treatmentcomparisons. The results for treatments with fewer than fiveobservational studies or five randomized, controlled trialsare summarized in Figure 1 and Figure 2. Results for treatmentswith more studies are shown in subsequent figures.
Figure 1. Results of Observational Studies and Randomized, Controlled Trials of Cardiologic Treatments.
The figure is based on data from eight articles.13,14,15,16,17,18,19,20 Some articles contain data from more than one study. OR denotes odds ratio, CI confidence interval, CAD coronary artery disease, CABG coronary-artery bypass graft surgery, PTCA percutaneous transluminal coronary angioplasty, CASS Coronary Artery Surgery Study, and Duke the Duke University Cardiovascular Disease Databank. Asterisks indicate studies that reported relative risks rather than odds ratios. Daggers indicate studies that reported neither a confidence interval nor a P value for the odds ratio.
Figure 2. Results of Observational Studies and Randomized, Controlled Trials of Noncardiologic Treatments.
The figure is based on data from 20 articles.21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40 Some articles contain data from more than one study. OR denotes odds ratio, and CI confidence interval. The asterisk indicates a study that reported relative risks rather than odds ratios.
Figure 1 summarizes the results of observational studies andcorresponding randomized, controlled trials for seven cardiologictreatments. The differences in design between the two typesof study were as follows: the dose of nifedipine in the observationalstudy was 30 to 60 mg, as compared with 30 to 50 mg in the randomized,controlled trials. The inclusion criteria and follow-up timesvaried among the randomized, controlled trials of nifedipine.For the observation-al study comparing coronary-artery bypassgrafting (CABG) with percutaneous transluminal coronary angioplasty(PTCA), low risk was defined by a proprietary Medisgroups scale.41
The observational results fell within the confidence intervalsof the randomized, controlled trials in every area except forthe comparison of CABG with PTCA in patients at low risk. Allof the other odds ratios were similar with the two study designs,except for the comparison of CABG and PTCA in diabetic patients.The confidence intervals of the observational studies were slightlynarrower than those of the randomized, controlled trials.
Figure 2 summarizes the results of observational studies andrandomized, controlled trials of 11 noncardiologic treatments.For the insulin studies, there was variation in inclusion criteria,and the follow-up times varied from 3 to 7.5 years. The resultsof all the observational studies fell within the confidenceintervals of the randomized, controlled trials, except for thecomparison of pneumatic retinopexy with scleral buckling. Theresults of the two types of study also differed qualitativelyfor three other treatments, although these differences are difficultto interpret because of the wide confidence intervals.
In one pseudo-randomized trial comparing water-soluble withoil-soluble contrast medium for flushing of ovarian tubes, theodds ratio for pregnancy was 2.00, as compared with 1.92 forboth the randomized, controlled trials and the observationalstudies.40 In one pseudo-randomized trial comparing geriatricassessment units and medical wards, the odds ratio for deathwas 0.51, as compared with 0.69 for the one observational studyand 0.65 for the randomized, controlled trials.42
Figure 3 shows the results of studies of the effects of onlyone treatment, hormone-replacement therapy, on lumbar bone mineraldensity after one to two years of treatment. The inclusion criteriafor the two types of study were identical, except for one randomized,controlled trial that included only women with osteoporosis.The randomized, controlled trials of this treatment, particularlythe later studies, had larger samples and narrower confidenceintervals. The combined result of the observational studieslay just below the lower bound of the confidence interval ofthe combined randomized, controlled trials, although the resultswere qualitatively very similar.
Figure 3. Percent Change in Lumbar Bone Density in Postmenopausal Women Given One to Two Years of Hormone-Replacement Therapy as Compared with Controls.
The figure is based on data from 15 articles.43,44,45,46,47,48,49,50,51,52,53,54,55,56,57 CI denotes confidence interval.
Figure 4 shows the results of studies evaluating the use ofcalcium-channel blockers in patients receiving kidney allografts.There were some differences in follow-up times among these studies:the follow-up time in the Morales study was only 30 days, andthe follow-up time in the Wahlberg and Ladefoged studies was3 months.63 All other follow-up times were between six monthsand two years. There were also differences in the immunosuppressiveregimens administered. The individual studies did not detecta significant effect of calcium-channel blockers, although themeta-analysis did. The overall results were almost identicalwith the two study designs.
Figure 4. Odds Ratio for Graft Survival after Kidney Transplantation in Patients Receiving Calcium-Channel Blockers as Compared with Controls.
The figure is based on data from six articles.58,59,60,61,62,63 The nine randomized, controlled trials were analyzed by Ladefoged and Andersen.63 OR denotes odds ratio, and CI confidence interval.
Figure 5 shows the results of studies comparing laparoscopicwith open appendectomy. This analysis involved 24 studies, thegreatest number of individual studies for any comparison. Fewof the individual studies demonstrated a significant benefitof laparoscopic appendectomy. However, the meta-analysis diddetect a benefit, which was of the same magnitude in observationalstudies and randomized, controlled trials.
Figure 5. Odds Ratio for Infection after Laparoscopic as Compared with Open Appendectomy.
The figure is based on data from nine articles.64,65,66,67,68,69,70,71,72 The 16 randomized, controlled trials were analyzed by Golub et al.72 Trials that reported rates of all complications rather than rates of infection have been excluded. OR denotes odds ratio, and CI confidence interval.
In general, for any specific treatment, the observational studieswere conducted before the results of the randomized, controlledtrials became available. However, in some cases the observationalstudies were conducted after the randomized, controlled trials.Reasons given for conducting later observational studies includeda lack of long-term safety data,14,46 concurrent collectionof the observational data from patients who declined to be includedin a randomized, controlled trial,17,38,39 and evaluation ofthe generalizability of the results of the randomized, controlledtrial in a wider population.16
Discussion
In this study we compared the results of observational studiesand randomized, controlled trials. We found 136 articles in19 treatment areas. All but six of these articles were publishedbetween 1985 and 1998. The estimates of the effects of treatmentin observational studies and in randomized, controlled trialswere similar in most areas, and for only 2 of the 19 treatmentsdid the magnitude of the effect in the combined observationalstudies lie outside the 95 percent confidence interval for thecombined randomized, controlled trials. For most treatments,however, there were insufficient data to exclude the possibilityof clinically important differences between the results of thetwo types of study.
The small number of suitable articles we found may be due partlyto limitations of computerized searches for reports of observationalstudies and partly to the paucity of treatments that have beenevaluated by both randomized, controlled trials and observationalstudies. Our results may not apply to other treatments. However,because the treatments evaluated were diverse, it is likelythat randomized, controlled trials and observational studies(at least those reported since 1985 in journals listed in theAbridged Index Medicus) often produce similar results.
There were discrepancies between the confidence intervals ofthe observational study and the randomized, controlled trialthat compared CABG with PTCA for patients at low risk. The mortalityrates were similar for the two treatments in the randomized,controlled trial, but in the observational study the mortalityrates were higher for the patients undergoing CABG, particularlyduring the first 60 days after surgery.73 In the randomized,controlled trial, patients at low risk undergoing CABG had verylow early mortality. This mortality rate may not be representativeof the mortality rate associated with CABG in most communityhospitals.74
The greatest statistical discrepancy between the results ofthe two types of study was for studies comparing pneumatic retinopexywith scleral buckling for the treatment of retinal detachment.The observational studies and the randomized, controlled trialboth found that the two procedures were associated with similarfinal rates of reattachment after reoperation and similar ratesof postoperative proliferative vitreoretinopathy.24,25,26 However,the randomized, controlled trial, but not the observationalstudies, found that the two procedures were associated withsimilar rates of reattachment after the first operation andthat pneumatic retinopexy had a better visual outcome than scleralbuckling. The results for patients undergoing scleral bucklingwere similar for the two study designs. One possible explanationfor these results is that the patients undergoing retinopexyin the observational studies were at higher risk than thoseundergoing scleral buckling. A more likely explanation, however,is that the outcome of retinopexy was unusually good in therandomized, controlled trial.
We did not select articles to reduce the heterogeneity of theresults or to ensure high quality (except that articles fromjournals listed in the Abridged Index Medicus were includedin every treatment comparison). The choice of selection criteriawas subjective and may have affected the results.75 On the otherhand, our results may have been influenced by the inclusionof flawed studies.
Our finding that observational studies and randomized, controlledtrials usually produce similar results differs from the conclusionsof previous authors. A study in 1977 reviewed the evidence ofthe effectiveness of anticoagulants in the treatment of acutemyocardial infarction, using eight observational studies andsix randomized, controlled trials.5 The differences in mortalityrates between control and treatment groups were larger in theobservational studies than in the randomized, controlled trials.The observational studies reviewed were published before 1975,and the authors did not use current meta-analytic techniquesfor pooling data. The results of the comparison might have differedif current methods had been used to combine the results of severaltrials.
Some of the same authors later reviewed 160 studies that evaluatedsix cardiology treatments.6 They found that the reported outcomeswere better for the treatment group than the controls in 60percent of randomized, controlled trials and 93 percent of observationalstudies. As pointed out at the time, however, most of theirstudies of beta-blockers were randomized, controlled trials,whereas most of their studies evaluating treatment in coronarycare units were observational.76 The greater treatment effectsin the observational studies might be explained by the greatereffectiveness of treatment in coronary care units than of treatmentwith beta-blockers.
Three other studies commonly cited to show the inadequacy ofobservational data,7,8,9 as well as one that found no bias inobservational data,77 also compared observational studies andrandomized, controlled trials that evaluated different treatments.As compared with these previous studies, our study has the advantagethat the comparisons were stratified according to treatment.In addition, the studies that we reviewed were more recent andtherefore may have used better methods than those in the earlierreviews.
A recent investigation to compare observational studies andrandomized, controlled trials was performed by the United KingdomHealth Technology Assessment Group.13 They found eight treatments,not evaluated by us, that were the subject of a randomized,controlled trial and of an observational study with a controlgroup. In seven of these there were no differences between theresults of the observational studies and the results of therandomized, controlled trials, and in the other the effect wasgreater in the observational studies. For the last treatmentcomparison (cost savings associated with hospice care),78 thelength of time in the hospice differed between the two typesof studies. The study by the United Kingdom Health TechnologyAssessment Group concluded that there were no systematic biasesin observational studies.
Although observational studies may generally give valid results,there are known limitations. In particular, as found by Greenand Byar,79 observational studies cannot be used to evaluatetreatments that physicians routinely select for the sickestpatients. On the basis of our findings, this misuse of observationalstudies does not often occur in the recent literature listedin the Abridged Index Medicus.
The fundamental criticism of observational studies is that unrecognizedconfounding factors may distort the results. According to theconventional wisdom, this distortion is sufficiently commonand unpredictable that observational studies are not reliableand should not be funded. Our results suggest that observationalstudies usually do provide valid information. They could beused to exploit the many recently developed, clinically richdata bases. Only with a greater willingness to analyze thesedata bases is it possible to achieve a realistic understandingof how observational studies can best be used.
Supported in part by grants from the Health Services and ResourcesAdministration (PD15 PE87007 and 5D32PE10195-02) and the NationalHeart, Lung, and Blood Institute (2T35HL07485-21).
We are indebted to Dedra Diehl, M.L.S., for help with libraryresearch.
Source Information
From the Department of Family Medicine, University of Iowa College of Medicine, Iowa City.
Address reprint requests to Dr. Hartz at the Department of Family Medicine, University of Iowa College of Medicine, 01292-D PFP, Iowa City, IA 52242-1097, or at arthur-hartz{at}uiowa.edu.
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