Journal of General Internal Medicine

, Volume 7, Issue 6, pp 623–629 | Cite as

The unit of analysis error in studies about physicians’ patient care behavior

  • George W. Divine
  • J. Trig Brown
  • Linda M. Frazier


Objective:To estimate the frequency with which patients are incorrectly used as the unit of analysis among statistical calculations in published studies of physicians’ patient care behavior.

Design:Retrospective review of studies published during 1980–1990.

Articles:54 articles retrieved by a computerized search using medical subject beadings for physicians and study characteristics. Article selection criteria included the requirement that the physician should have been the correct unit of analysis.

Intervention:Presence of the error was determined by consensus using published criteria.

Main results:The error was present in 38 articles (70%). The number of study physicians was reported in 35 articles (65%). The error was found in 57% of articles that reported the number of study physicians and in 95% of those that did not. The error rate was not lower among articles published more recently nor among those published in journals with higher rates of article citations in the medical literature.

Conclusion:The unit of analysis error occurs frequently and can generate artificially low p values. Failure to report the number of study physicians can be a clue that this type of error has been made.

Key words

data analysis, statistical physicians education patient outcome assessment quality assurance 


  1. 1.
    Whiting-O’Keefe QE, Henke C, Simborg DW. Choosing the correct unit of analysis in medical care experiments. Med Care. 1984;22:1101–14.PubMedCrossRefGoogle Scholar
  2. 2.
    Kleinbaum DG, Kupper LL, Muller KE. Applied regression analysis and other multivariable methods, 2nd ed. Boston: PWS Kent, 1988.Google Scholar
  3. 3.
    Remington RD, Schork MA. Statistics with applications to the biological and health sciences, 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1985.Google Scholar
  4. 4.
    Garfield E, (ed.). SCI journal citation reports. Philadelphia: Institute for Scientific Information, 1988;19:3–42.Google Scholar
  5. 5.
    Snedecor GW, Cochran WG. Statistical methods, 7th ed. Ames, IA: Iowa State University Press, 1980.Google Scholar
  6. 6.
    Scheffe H. The analysis of variance. New York: John Wiley and Sons, 1985.Google Scholar
  7. 7.
    Freiman JA, Chalmers TC, Smith HC, Keubler RR. The importance of beta, the type II error and sample size in the design and interpretation of the randomized control trial: survey of 71 “negative” trials. N Engl J Med. 1978;299:690–4.PubMedCrossRefGoogle Scholar
  8. 8.
    Godfrey K. Statistics in practice: comparing the means of several groups. N Engl J Med. 1985;313:1450–6.PubMedCrossRefGoogle Scholar
  9. 9.
    Pocock SJ, Hughes MD, Lee RJ. Statistical problems in the reporting of clinical trials: a survey of three medical journals. N Engl J Med. 1987;317:426–32.PubMedCrossRefGoogle Scholar
  10. 10.
    McKinney WP, Young MJ, Hartz A, Lee MB-F. The inexact use of Fisher’s exact test in six major medical journals. JAMA. 1989;261:3430–3.PubMedCrossRefGoogle Scholar
  11. 11.
    Wallenstein S, Zucker CL, Fleiss JL. Some statistical methods useful in circulation research. Circ Res. 1980;47:1–9.PubMedGoogle Scholar

Appendix A Articles Evaluated for the Unit of Analysis Error, Sorted by Absence or Presence of the Error Correct Unit of Analysis

  1. 1.
    Cohen SJ, Christen AG, Katz BP, et al. Counseling medical and dental patients about cigarette smoking: the impact of nicotine gum and chart reminders. Am J Public Health. 1987;77:313–6.PubMedGoogle Scholar
  2. 2.
    Cohen SJ, Stookey GK, Katz BP, Drook CA, Smith DM. Encouraging primary care physicians to help smokers quit: a randomized, controlled trial. Ann Intern Med. 1989;110:648–52.PubMedGoogle Scholar
  3. 3.
    Jennett PA, Wilson TW, Hayton RC, Mainprize GW, Laxdal OE. Desirable behaviors in the office management of hypertension addressed through continuing medical education. Can J Public Health. 1989;80:359–62.PubMedGoogle Scholar
  4. 4.
    McAuley RG, Henderson HW. Results of the peer assessment program of the College of Physicians and Surgeons of Ontario. Can Med Assoc J. 1984;131:557–61.PubMedGoogle Scholar
  5. 5.
    Putnam RW, Curry L. Impact of patient care appraisal on physician behavior in the office setting. Can Med Assoc J. 1985;132:1025–9.PubMedGoogle Scholar
  6. 6.
    Bass MJ, McWhinney IR, Donner A. Do family physicians need medical assistants to detect and manage hypertension? Can Med Assoc J. 1986;134:1247–55.Google Scholar
  7. 7.
    Eisenberg JM, Williams SV. Limited usefulness of the proportion of tests with normal results in review of diagnostic services utilization. Clin Chem. 1983;29:2111–3.PubMedGoogle Scholar
  8. 8.
    Larsen KM, Kirkwood CR. Procedures performed by family physicians, internists and a Medex in a small group practice. J Fam Pract. 1982;15:285–92.PubMedGoogle Scholar
  9. 9.
    Maxwell JA, Sandlow LJ, Bashook PG. Effect of a medical care evaluation program on physician knowledge and performance. J Med Educ. 1984;59:33–8.PubMedGoogle Scholar
  10. 10.
    Dunn EV, Bass MJ, Williams JI, Borgiel AEW, MacDonald P, Spasoff RA. Study of the relation of continuing medical education to quality of family physicians’ care. J Med Educ. 1988;63:775–84.PubMedGoogle Scholar
  11. 11.
    White PT, Pharoah CA, Anderson HR, Freeling P. Randomized controlled trial of small group education on the outcome of chronic asthma in general practice. J R Coll Gen Pract. 1989;39:182–6.PubMedGoogle Scholar
  12. 12.
    Hartzema AG, Christensen DB. Nonmedical factors associated with the prescribing volume among family practitioners in an HMO. Med Care. 1983;21:990–1000.PubMedCrossRefGoogle Scholar
  13. 13.
    Gelbach SH, Wilkenson WE, Hammond WE, et al. Improving drug prescribing in a primary care practice. Med Care. 1984;22:193–201.CrossRefGoogle Scholar
  14. 14.
    Palmer RH, Louis TA, Hsu LN, et al. A randomized controlled trial of quality assurance in sixteen ambulatory care practices. Med Care. 1985;23:751–70.PubMedCrossRefGoogle Scholar
  15. 15.
    Sibley JC, Sackett DL, Neufeld V, Gerrard B, Rudnick KV, Fraser W. A randomized trial of continuing medical education. N Engl J Med. 1982;306:511–5.PubMedCrossRefGoogle Scholar
  16. 16.
    Rodney WM, Ruggiero C. Outcomes following continuing medical education on flexible sigmoidoscopy. Fam Pract 1987;4:306–10.PubMedCrossRefGoogle Scholar

Incorrect Unit of Analysis

  1. 17.
    Donowitz LG, Handwashing technique in a pediatric intensive care unit. Am J Dis Child. 1987;141:683–5.PubMedGoogle Scholar
  2. 18.
    Freedman MA, Jillson DA, Coffin RR, Novick LF. Comparison of complication rates in first trimester abortions performed by physician assistants and physicians. Am J Public Health. 1986;76:550–4.PubMedGoogle Scholar
  3. 19.
    Martin DP, Kiehr P, Price KF, Richardson WC. Effect of a gatekeeper plan on health services use and charges: a randomized trial. Am J Public Health. 1989;79:1628–32.PubMedCrossRefGoogle Scholar
  4. 20.
    McPhee SJ, Lo B, Saika GY, Metzler R. How good is communication between primary care physicians and subspecialty consultants? Arch Intern Med. 1984;144:1265–8.PubMedCrossRefGoogle Scholar
  5. 21.
    Sznajder JI, Zveibil FR, Bitterman H, Wiener P, Bursztein S. Central vein catheterization: failure and complication rates by three percutaneous approaches. Ann Intern Med. 1986;146:259–61.CrossRefGoogle Scholar
  6. 22.
    Brody DS, Lerman CE, Wolfson HG, Caputo GC. Improvement in physicians’ counseling of patients with mental health problems. Arch Intern Med. 1990;150:993–8.PubMedCrossRefGoogle Scholar
  7. 23.
    Dearlove J, Bate T, Dearlove B, Newman P. Ignoring the obvious: doctors’ wives as patients. BMJ. 1982;285:187–9.PubMedGoogle Scholar
  8. 24.
    Russell MAH, Stapleton JA, Jackson PH, Hajek P, Belcher M. District programme to reduce smoking: effect of clinic supported brief intervention by general practitioners. BMJ. 1987;295:1240–4.PubMedGoogle Scholar
  9. 25.
    Ried AJ, Carroll JC, Ruderman J, Murray MA. Differences in intrapartum obstetric care provided to women at low risk by family physicians and obstetricians. Can Med Assoc J. 1989;140:625–33.Google Scholar
  10. 26.
    Krikke EH, Bell NR. Relation of family physician or specialist care to obstetric interventions and outcomes in patients at low risk: a western Canadian cohort study. Can Med Assoc J. 1989;140:637–43.Google Scholar
  11. 27.
    Susman J, Zervanos NJ. Continuity of care and outcome in nursing home patients transferred to a community hospital. Fam Med. 1989;21:118–21.PubMedGoogle Scholar
  12. 28.
    Presser SE, Taylor JR. Clinical diagnostic accuracy of basal cell carcinoma. J Am Acad Dermatol. 1987;16:988–90.PubMedCrossRefGoogle Scholar
  13. 29.
    Chaska BW, Mellstrom MS, Grambsch PM, Nesse RE. Influence of site of obstetric care and delivery on pregnancy management and outcome. J Am Board Fam Pract. 1988;1:152–63.PubMedGoogle Scholar
  14. 30.
    Bowman MW, Gehlbach SH. Sex of physician as a determinant of psychosocial problem recognition. J Am Board Fam Pract. 1980;10:655–9.Google Scholar
  15. 31.
    Hamburger S, Barjenbruch P, Soffer A. Treatment of diabetic ketoacidosis by internists and family physicians: a comparative study. J Fam Pract. 1982;14:719–22.PubMedGoogle Scholar
  16. 32.
    Frame PS, Kowulich BA, Llewellyn AM. Improving physician compliance with a health maintenance protocol. J Fam Pract. 1984;19:341–4.PubMedGoogle Scholar
  17. 33.
    Rodney WM, Beaber RJ, Johnson R, Quan M. Physician compliance with colorectal cancer screening (1978 – 1983): the impact of flexible sigmoidoscopy. J Fam Pract. 1985;20:265–9.PubMedGoogle Scholar
  18. 34.
    Halvorsen JG, Kunian A, Gjerdingen D, et al. The interpretation of office radiographs by family physicians. J Fam Pract. 1989;28:426–32.PubMedGoogle Scholar
  19. 35.
    White RH, Hong R, Venook AP, et al. Initiation of warfarin therapy: comparison of physician dosing with computer-assisted dosing. J Gen Intern Med. 1987;2:141–8.PubMedCrossRefGoogle Scholar
  20. 36.
    Cohen D, Berner U, Dubach UC. Physician compliance in the management of hypertensive patients. J Hypertens. 1985;3:73–6.CrossRefGoogle Scholar
  21. 37.
    Rodney WM, Chopivsky P, Quan M. Adult immunization: the medical record design as a facilitator for physician compliance. J Med Educ. 1983;58:576–80.PubMedGoogle Scholar
  22. 38.
    Wones RG, Rouan GW, Brody TL, Bode RB, Radack KL. An ambulatory medical education program for internal medicine residents. J Med Educ. 1987;62:470–6.PubMedGoogle Scholar
  23. 39.
    Baxt WG, Moody P. The impact of a physician as part of the aeromedical prehospital team in patients with blunt trauma. JAMA. 1987;257:3246–50.PubMedCrossRefGoogle Scholar
  24. 40.
    Wilson DM, Taylor DW, Gilbert JR, et al. A randomized trial of a family physician intervention for smoking cessation. JAMA. 1988;260:1570–4.PubMedCrossRefGoogle Scholar
  25. 41.
    Dusitsin N, Varakamin S, Ningsanon P, Chalapati S, Boonsiri B, Gray RH. Post-partum tubal ligation by nurse-midwives and doctors in Thailand. Lancet. 1980;1:638–9.PubMedCrossRefGoogle Scholar
  26. 42.
    McGhan WF, Stimmel GL, Hall TG, Gilman TM. A comparison of pharmacists and physicians on the quality of prescribing for ambulatory hypertensive patients. Med Care. 1983;21:435–44.PubMedCrossRefGoogle Scholar
  27. 43.
    Thompson RS, Michnich ME, Gray J, Friedlander L, Gilson B. Maximizing compliance with hemoccult screening for colon cancer in clinical practice. Med Care 1986;24:904–14.PubMedCrossRefGoogle Scholar
  28. 44.
    McDowell I, Newell C, Rosser W. A randomized trial of computerized reminders for blood pressure screening in primary care. Med Care. 1989;27:297–305.PubMedCrossRefGoogle Scholar
  29. 45.
    Copeman RC, Swannell RJ, Pincus DF, Woodhead KA. Utilization of the “Smokescreen” smoking-cessation programme by general practitioners and their patients. Med J Aust. 1989;151:83–7.PubMedGoogle Scholar
  30. 46.
    Foote A, Erfurt JC. Hypertension control at the work site: comparison of screening and referral alone, referral and follow-up, and on-site treatment. N Engl J Med. 1983;308:809–13.PubMedCrossRefGoogle Scholar
  31. 47.
    Tilyard MW, Williams S, Seddon RJ, Oakley ME, Murdoch JC. Is outcome for low risk obstetric patients influenced by parity and intervention? N Z Med J. 1989;102:523–6.PubMedGoogle Scholar
  32. 48.
    Leduc DG, Pless IB. Pediatricians and general practitioners: a comparison of the management of children with febrile illness. Pediatrics. 1982;70:511–5.PubMedGoogle Scholar
  33. 49.
    Li VC, Kim YJ, Ewart CK, et al. Effects of physician counseling on the smoking behavior of asbestos-exposed workers. Prev Med. 1984;13:462–76.PubMedCrossRefGoogle Scholar
  34. 50.
    Hawton K, McKeown S, Day A, Martin P, O’Connor M, Yule J. Evaluation of out-patient counselling compared with general practitioner care following overdoses. Psychol Med. 1987;17:751–61.PubMedCrossRefGoogle Scholar
  35. 51.
    Vandvik LH, Hoyeraal HM, Larsen S. Agreement between parents and physicians regarding clinical evaluation of patients with juvenile rheumatoid arthritis. Scand J Rheumatol. 1988;17:459–63.PubMedGoogle Scholar
  36. 52.
    Irwig LM, Porter B, Wilson TD, et al. Clinical competence of paediatric primary health care nurses in Soweto. S Afr Med J. 1985;67:92–5.PubMedGoogle Scholar
  37. 53.
    Oleynikov SP, Glasunov IS, Chazova LV. Results of one-year antismoking campaign conducted within the framework of multifactorial prevention of ischaemic heart disease. Cor Vasa. 1982;24:318–28.PubMedGoogle Scholar
  38. 54.
    Timpka T, Bjurulf P, Burr T. Audit of decision-making regarding female genitourinary infections in outpatient practice. Scand J Infect Dis. 1990;22:49–57.PubMedGoogle Scholar

Copyright information

© Society of General Internal Medicine 1992

Authors and Affiliations

  • George W. Divine
    • 1
  • J. Trig Brown
    • 3
  • Linda M. Frazier
    • 2
  1. 1.the Department of Community and Family Medicine, Division of Biometry and Medical InformaticsDuke UniversityDurham
  2. 2.Division of Occupational and Environmental MedicineDuke UniversityDurham
  3. 3.the Department of Medicine, Division of General Internal MedicineDuke UniversityDurham
  4. 4.Henry Ford Health SystemDetroit
  5. 5.Southeast Permanente Medical GroupDurham
  6. 6.Henry Ford Health System, Division of BiostatisticsResearch Epidemiology and ComputingSouthfield

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