at the Department of Community Medicine, University College of Medical Sciences. It is being distributed under a CC – BY – NC (Attribution-Non-Commercial 3.0 Unported) License 2 Estimating Risk: RR, OR, Adjusted OR by Pranab Chatterjee is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.
B: – Risk of developing disease: 10% • Difference in risk: what is the absolute excess risk present in Group B? – 10% - 5% = 5 percent points • Ratio: how many times larger is the risk in Group B? – 10%/5% = 2 11
not a frequent one • When the cases studied are representative of all people with the disease in the population from which the cases were drawn, with regards to history of the exposure • When the controls studied are representative of all the people without the disease in the population from which the cases were drawn, with regards to history of exposure 26
occurs can be attributed to a certain exposure? • It is the amount or proportion of disease incidence (or disease risk) that can be attributed to a specific exposure. • Eg: How much of the Lung CA risk can be attributed to smoking? 50 Gordis L. Epidemiology. Edition 4
• Incidence of CHD in non smokers = 1/1000 • 45% of the total population smokes – Incidence of CHD attributable to smoking in the total population? – Proportion of the risk in the total population attributable to smoking? 55 Open Courseware: Johns Hopkins School of Public Health: http://ocw.jhsph.edu
n Level Low High Low High Low 1.0 7.7 1.0 9.7 High 18.2 146.8 6.2 14.2 72 Relative Risks of Lung CA based on smoking levels in two radiation exposed populations Blot WJ, Akiba S, Kato H. Ionising radiation and lung cancer: A review including preliminary results from a case control study among A-bomb survivors. In Prentice RL, Thomson DJ (eds): Atomic Bomb survivor data: Utilization and analysis. Philadelphia, Society for industrial and applied mathematics. 1984, pp235-48.
value, then the odds ratio becomes zero for that strata • And log (0) is undefined • Invalidates the stratification • However: – Easy to understand – Small number of strata – Large sample size in each strata 75
1959 • Weighted mean of the odds ratio of the individual strata. (hence “common”) 79 Mantel, N.; Haenszel, W. (1959), "Statistical aspects of the analysis of data from the retrospective analysis of disease", Journal of the National Cancer Institute 22 (4): 719–748, PMID 13655060
of odds ratio divided by the total sample size at the strata) • Denominator: summation of (denominator component of odds ratio divided by the total sample size at the strata) 82 ˆ θMH = n k11 n k00 / n k k=1 r ∑ n k10 n k01 / n k k=1 r ∑
CI = 0.19 – 0.72 83 N=255 HIV + HIV - AZT 0 91 No AZT 3 161 N=92 HIV + HIV - AZT 8 40 No AZT 16 28 • Numerator: summation of (numerator component of odds ratio divided by the total sample size at the strata) • Denominator: summation of (denominator component of odds ratio divided by the total sample size at the strata)
to “visualize” • Clinical use of OR limited • Misreporting of OR in published papers • OR: A Ratio of Ratios! • The RR = OR conundrum 86 Holcomb WL Jr, Chaiworapongsa T, Luke DA, Burgdorf KD. An odd measure of risk: use and misuse of the odds ratio. Obstet Gynecol 2001;98:685–8. Katz KA. The (relative) risks of using odds ratios. Arch Dermatol 2006;142: 761–4.