Biostatistics for psychiatry exams

Meet the exam question

The examiner puts one sentence in front of you, lifted from an abstract: "Adjunctive treatment produced a 43% relative improvement in response at eight weeks (p = 0.04)." Then: "Would you change your prescribing?"[3][8]

There is exactly one correct first move, and it is not an opinion. It is a question: what were the two event rates? Because 43% relative on a control response of 35% means the drug moved 35 out of 100 to 50 out of 100 — an absolute gain of 15 percentage points, NNT 7. The same 43% on a control response of 3.5% would mean an absolute gain of 1.5 points and NNT 67. Identical headline, two different clinics.[7][8]

Psychiatry is a hostile numerical environment: subjective rating scales, large placebo response, modest samples, many endpoints, and sparse hard events like suicide and relapse. Every one of those inflates false positives and widens intervals. This topic is the arithmetic that stops you being fooled.[3][18]

[1] [2] [5]

References22Show ledgerHide ledger

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2. [2]Greenland S, Senn SJ, Rothman KJ, et al. Statistical tests, P values, confidence intervals, and power: a guide to misinterpretations Eur J Epidemiol, 2016.PMID 27209009
3. [3]Sterne JA, Davey Smith G Sifting the evidence—what's wrong with significance tests? BMJ, 2001.PMID 11159626
4. [4]Altman DG, Bland JM Absence of evidence is not evidence of absence BMJ, 1995.PMID 7647644
5. [5]Freiman JA, Chalmers TC, Smith H Jr, Kuebler 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.PMID 355881
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10. [10]Grimes DA, Schulz KF Making sense of odds and odds ratios Obstet Gynecol, 2008.PMID 18238982
11. [11]Spruance SL, Reid JE, Grace M, Samore M Hazard ratio in clinical trials Antimicrob Agents Chemother, 2004.PMID 15273082
12. [12]Altman DG, Bland JM Diagnostic tests. 1: Sensitivity and specificity BMJ, 1994.PMID 8019315
13. [13]Altman DG, Bland JM Diagnostic tests 2: Predictive values BMJ, 1994.PMID 8038641
14. [14]Altman DG, Bland JM Diagnostic tests 3: receiver operating characteristic plots BMJ, 1994.PMID 8044101
15. [15]Deeks JJ, Altman DG Diagnostic tests 4: likelihood ratios BMJ, 2004.PMID 15258077
16. [16]Hanley JA, McNeil BJ The meaning and use of the area under a receiver operating characteristic (ROC) curve Radiology, 1982.PMID 7063747
17. [17]Bland JM, Altman DG Survival probabilities (the Kaplan-Meier method) BMJ, 1998.PMID 9836663
18. [18]Ioannidis JPA Why most published research findings are false PLoS Med, 2005.PMID 16060722
19. [19]Guyatt GH, Oxman AD, Vist GE, et al. GRADE: an emerging consensus on rating quality of evidence and strength of recommendations BMJ, 2008.PMID 18436948
20. [20]Jaeschke R, Guyatt GH, Sackett DL Users' guides to the medical literature. III. How to use an article about a diagnostic test. B. What are the results and will they help me in caring for my patients? JAMA, 1994.PMID 8309035
21. [21]Bossuyt PM, Reitsma JB, Bruns DE, et al. STARD 2015: an updated list of essential items for reporting diagnostic accuracy studies BMJ, 2015.PMID 26511519
22. [22]Higgins JP, Thompson SG, Deeks JJ, Altman DG Measuring inconsistency in meta-analyses BMJ, 2003.PMID 12958120