| Author(s) | Kunz R; Oxman AD
|
|---|
| Title | The unpredictability paradox: review of empirical comparisons of randomised and non-randomised clinical trials
|
|---|
| Source | BRITISH MEDICAL JOURNAL 317 (7167): 1185-1190
|
|---|
| Date | 1998 OCT 31
|
|---|
| Type | Journal : Article
|
|---|
| LCR: 1 NCR: 42 LCS: 2 GCS: 169
| | Comment |
|
|---|
| Address | Natl Inst Publ Hlth, Hlth Serv Res Unit, Oslo, Norway.
Charite, Dept Nephrol, Berlin, Germany.
|
|---|
| Reprint | Oxman, AD, Natl Inst Publ Hlth, Hlth Serv Res Unit, Oslo, Norway.
|
|---|
| E-mail | andrew.oxman@labmed.uio.no
|
|---|
| Abstract | Objective To summarise comparisons of randomised clinical trials and non-randomised clinical trials, trials with adequately concealed random allocation versus inadequately concealed random allocation, and high quality trials versus low quality trials where the effect of randomisation could not be separated from the effects of other methodological manoeuvres. Design Systematic review Selection criteria Cohorts or meta-analyses of clinical trials that included an empirical assessment of the relation between randomisation and estimates of effect Data sources Cochrane Review Methodology Database, Medline, SciSearch, bibliographies, hand searching of journals, personal communication with methodologists, and the reference lists of relevant articles. Main outcome measures Relation between randomisation and estimates of effect Results Eleven studies that compared randomised controlled trials with non-randomised controlled trials (eight for evaluations of the same intervention and three across different interventions), two studies that compared trials with adequately concealed random allocation and inadequately concealed random allocation, and five studies that assessed the relation between quality scores and estimates of treatment effects, were identified. Failure to use random allocation and concealment of allocation were associated with relative increases in estimates of effects of 150% or more, relative decreases of up to 90%, inversion of the estimated effect and, in some cases, no difference. On average, failure to use randomisation or adequate concealment of allocation resulted in larger estimates of effect due to a poorer prognosis in non-randomly selected control groups compared with randomly selected control groups. Conclusions Failure to use adequately concealed random allocation can distort the apparent effects of care in either direction, causing the effects to seem either larger or smaller than they really are. The size of these distortions can be as large as or larger than the size of the effects that are to be detected.
|
|---|
| CR |
*COCHR EMP METH ST, 1998, COCHR LIBR
*COCHR METH WORK G, 1998, COCHR LIBR
*COMM EV MED TECHN, 1985, ASS MED TECHN, P76
*NHS CTR REV DISS, 1998, COCHR LIBR
*REC MISC IMM TRIA, 1994, AM J REPROD IMMUNOL, V32, P55
*US C, 1994, OTATCT600, P41
BLACK N, 1996, BRIT MED J, V312, P1215
CARROLL D, 1996, BRIT J ANAESTH, V77, P798
CHALMERS I, 1997, J ROY SOC MED, V90, P379
CHALMERS TC, 1977, NEW ENGL J MED, V297, P1091
CHALMERS TC, 1983, NEW ENGL J MED, V309, P1358
CLARKE M, 1998, COCHRANE LIBRARY
COCHRANE AL, 1972, EFFECTIVENESS EFFICI, P20
COLDITZ GA, 1989, STAT MED, V8, P441
COLDITZ GA, 1994, JAMA-J AM MED ASSOC, V271, P698
COUNSELL CE, 1994, BRIT MED J, V309, P1677
DANS AL, 1998, JAMA-J AM MED ASSOC, V279, P545
DICKERSIN K, 1997, AIDS EDUC PREV A, V9, P15
DICKSERSIN K, 1993, ONLINE J CURR CLIN T
DIEHL LF, 1986, J CLIN ONCOL, V4, P1114
EGGER M, 1997, BRIT MED J, V315, P629
EMERSON JD, 1990, CONTROL CLIN TRIALS, V11, P339
FORGIE MA, 1998, ARCH INTERN MED, V158, P610
GUYATT GH, 1993, JAMA-J AM MED ASSOC, V270, P2598
GUYATT GH, 1994, JAMA-J AM MED ASSOC, V271, P59
IAONNIDIS JPA, 1998, JAMA-J AM MED ASSOC, V279, P281
IMPERIALE TF, 1990, ANN INTERN MED, V113, P299
KHAN KS, 1996, ARCH INTERN MED, V156, P661
KLEIJNEN J, 1997, NONRANDOM REFLECTION, P93
MILLER JN, 1989, STAT MED, V8, P455
NURMOHAMED MT, 1992, LANCET, V340, P152
ORTIZ Z, 1998, J RHEUMATOL, V25, P36
OTTENBACHER K, 1992, CONTROL CLIN TRIALS, V13, P50
OXMAN A, 1998, EVIDENCE BASED PRACT, P91
PYORALA S, 1995, J CLIN ENDOCR METAB, V80, P2795
REIMOLD SC, 1992, AM HEART J, V124, P924
SACKS H, 1982, AM J MED, V72, P233
SCHULZ KF, 1995, JAMA-J AM MED ASSOC, V273, P408
STERN JM, 1997, BRIT MED J, V315, P640
STIEB DM, 1990, CAN MED ASSOC J, V142, P719
WATSON A, 1994, FERTIL STERIL, V61, P470
WEISS CH, 1998, METHODS STUDYING PRO, P229
|
|---|
|