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Chapter 11 - Understanding P-Values and Confidence Intervals

Published online by Cambridge University Press:  02 May 2020

Thomas B. Newman
University of California, San Francisco
Michael A. Kohn
University of California, San Francisco
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In the previous two chapters, we discussed using the results of randomized trials and observational studies to estimate treatment effects. We were primarily interested in measures of effect size and in problems with design (in randomized trials) and confounding (in observational studies) that could bias effect estimates. We did not focus on whether the apparent treatment effects could be a result of chance or attempt to quantify the precision of our effect estimates. The statistics used to help us with these issues − P-values and confidence intervals – are the subject of this chapter.

Evidence-Based Diagnosis
An Introduction to Clinical Epidemiology
, pp. 280 - 302
Publisher: Cambridge University Press
Print publication year: 2020

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Prevail II Writing Group; Multi-National, Prevail II. Study Team, Davey, RT Jr., Dodd, L, Proschan, MA, et al. A randomized, controlled trial of ZMapp for Ebola virus infection. N Engl J Med. 2016;375(15):1448–56.Google ScholarPubMed
Laptook, AR, Shankaran, S, Tyson, JE, et al. Effect of therapeutic hypothermia initiated after 6 hours of age on death or disability among newborns with hypoxic-ischemic encephalopathy: a randomized clinical trial. JAMA. 2017;318(16):1550–60.CrossRefGoogle ScholarPubMed
Center for Devices and Radiological Health FaDA. Guidance for the use of Bayesian statistics in medical device clinical trials. 2010. Available from: accessed September 23, 2019.Google Scholar
Greenland, S, Senn, SJ, Rothman, KJ, et al. Statistical tests, P values, confidence intervals, and power: a guide to misinterpretations. Eur J Epidemiol. 2016;31(4):337–50.CrossRefGoogle ScholarPubMed
Goodman, S. A dirty dozen: twelve p-value misconceptions. Semin Hematol. 2008;45(3):135–40.CrossRefGoogle ScholarPubMed
Goodman, SN. Of P-values and Bayes: a modest proposal. Epidemiology. 2001;12(3):295–7.CrossRefGoogle ScholarPubMed
Goodman, SN. Toward evidence-based medical statistics. 2: the Bayes factor. Ann Intern Med. 1999;130(12):1005–13.Google ScholarPubMed
Goodman, SN. Toward evidence-based medical statistics. 1: the P value fallacy. Ann Intern Med. 1999;130(12):9951004.CrossRefGoogle ScholarPubMed
Yudkowski, E. Cognitive biases potentially affecting judgment of global risks. In: Bostrom, N, Cirkovic, M, editors. Global catastrophic risks. New York: Oxford University Press; 2008. pp. 91119.Google Scholar
Schlosser, E. Command and control: nuclear weapons, the Damascus accident, and the illusion of safety. New York: The Penguin Press; 2013. xxiii, 632pp.Google Scholar
Ehrlich, PR, Ehrlich, AH. Can a collapse of global civilization be avoided? Proc Biol Sci. 2013;280(1754):20122845.Google ScholarPubMed
Diamond, J. Collapse: how societies choose to fail or succeed. New York: Viking Press; 2005.Google Scholar
Browner, WS, Newman, TB. Are all significant P values created equal? The analogy between diagnostic tests and clinical research. JAMA. 1987;257(18):2459–63.CrossRefGoogle ScholarPubMed
Feynman, R, Leighton, R, Sands, M. Six easy pieces: essentials of physics explained by its most brilliant teacher. New York: Basic Books; 2011.Google Scholar
Cao, J, Zhang, S. Multiple comparison procedures. JAMA. 2014;312(5):543–4.CrossRefGoogle ScholarPubMed
Yadav, K, Lewis, RJ. Gatekeeping strategies for avoiding false-positive results in clinical trials with many comparisons. JAMA. 2017;318(14):1385–6.CrossRefGoogle ScholarPubMed
Storey, JD. The positive false discovery rate: a Bayesian interpretation and the q-value. Ann Stat. 2003;31(6):2013–35.CrossRefGoogle Scholar
Guyatt, G, Rennie, D, Evidence-Based Medicine Working Group, American Medical Association. Users’ guides to the medical literature: a manual for evidence-based clinical practice. Chicago: AMA Press; 2002. xxiii, 706pp.Google Scholar
PREVAIL II Writing Group, Multi-National PIST, Davey, RT Jr., Dodd, L, Proschan, MA, et al. A randomized, controlled trial of ZMapp for Ebola virus infection. N Engl J Med. 2016;375(15):1448–56.Google ScholarPubMed
Dodd, LE, Proschan, MA, Neuhaus, J, et al. Design of a randomized controlled trial for Ebola virus disease medical countermeasures: PREVAIL II, the Ebola MCM study. J Infect Dis. 2016;213(12):1906–13.CrossRefGoogle ScholarPubMed
Lee, JJ, Chu, CT. Bayesian clinical trials in action. Stat Med. 2012;31(25):2955–72.Google ScholarPubMed
Sackett, D, Haynes, R, Guyatt, G, Tugwell, P. Clinical epidemiology: a basic science for clinical medicine. Boston: Little, Brown and Company; 1991. 52pp.Google Scholar
Jaffe, DM, Tanz, RR, Davis, AT, Henretig, F, Fleisher, G. Antibiotic administration to treat possible occult bacteremia in febrile children. N Engl J Med. 1987;317(19):1175–80.CrossRefGoogle ScholarPubMed
Newman, TB, Pantell, RH. Occult bacteremia in febrile children. N Engl J Med. 1988;318(20):1338–9.Google Scholar
Hanley, JA, Lippman-Hand, A. If nothing goes wrong, is everything all right? Interpreting zero numerators. JAMA. 1983;249(13):1743–5.CrossRefGoogle ScholarPubMed
Newman, TB. If almost nothing goes wrong, is almost everything all right? Interpreting small numerators. JAMA. 1995;274(13):1013.CrossRefGoogle ScholarPubMed
Keller, MB, Ryan, ND, Strober, M, et al. Efficacy of paroxetine in the treatment of adolescent major depression: a randomized, controlled trial. J Am Acad Child Adolesc Psychiatry. 2001;40(7):762–72.CrossRefGoogle ScholarPubMed
Le Noury, J, Nardo, JM, Healy, D, et al. Restoring Study 329: efficacy and harms of paroxetine and imipramine in treatment of major depression in adolescence. BMJ. 2015;351:h4320.CrossRefGoogle ScholarPubMed
Brown, HK, Ray, JG, Wilton, AS, et al. Association between serotonergic antidepressant use during pregnancy and autism spectrum disorder in children. JAMA. 2017;317(15):1544–52.CrossRefGoogle ScholarPubMed
Prymula, R, Siegrist, CA, Chlibek, R, et al. Effect of prophylactic paracetamol administration at time of vaccination on febrile reactions and antibody responses in children: two open-label, randomised controlled trials. Lancet. 2009;374(9698):1339–50.CrossRefGoogle ScholarPubMed
Veronesi, U, Paganelli, G, Viale, G, et al. A randomized comparison of sentinel-node biopsy with routine axillary dissection in breast cancer. N Engl J Med. 2003;349(6):546–53.CrossRefGoogle ScholarPubMed
Krag, D, Ashikaga, T. The design of trials comparing sentinel-node surgery and axillary resection. N Engl J Med. 2003;349(6):603–5.CrossRefGoogle ScholarPubMed
Krag, DN, Anderson, SJ, Julian, TB, et al. Sentinel-lymph-node resection compared with conventional axillary-lymph-node dissection in clinically node-negative patients with breast cancer: overall survival findings from the NSABP B-32 randomised phase 3 trial. Lancet Oncol. 2010;11(10):927–33.CrossRefGoogle ScholarPubMed
Giuliano, AE, Ballman, KV, McCall, L, et al. Effect of axillary dissection vs no axillary dissection on 10-year overall survival among women with invasive breast cancer and sentinel node metastasis: the ACOSOG Z0011 (Alliance) randomized clinical trial. JAMA. 2017;318(10):918–26.CrossRefGoogle ScholarPubMed

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