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Chapter-15 Assessing risk

BOOK TITLE: Research Methodology Simplified: Every Clinician a Researcher

Author
1. Parikh Mahendra N
2. Mukherjee Joydev
3. Hazra Avijit
4. Gogtay Nithya
ISBN
9789350250037
DOI
10.5005/jp/books/11435_15
Edition
1/e
Publishing Year
2010
Pages
7
Author Affiliations
1. Seth GS Medical College and Nowrosjee Wadia Maternity Hospital, Mumbai, Seth Gordhandas Sunderdas Medical College, Nowrosjee Wadia Maternity Hospital, Mumbai, Maharashtra, India; Shushrusha Citizens’ Cooperative Hospital, Mumbai, Maharashtra, India; Fertility Sterility, India; The Journal of Obstetrics and Gynaecology of India, Nowrosjee Wadia Maternity Hospital, Mumbai, Mumbai, Maharashtra, India, Mumbai, Seth GS Medical College and Nowrosjee Wadia Maternity Hospital, Mumbai, Maharashtra, India
2. North Bengal Medical College, West Bengal, India, RG Kar Medical College, Kolkota, RG Kar Medical College, Kolkata, India, RG Kar Medical College, Kolkata, RG Kar Medical College, Kolkata, West Bengal, India
3. Institute of Postgraduate Medical Education, and Research, Kolkata, Institute of Postgraduate Medical Education and Research, Kolkata, India, Institute of Postgraduate Medical Education and Research (IPGMER), Kolkata, West Bengal, India
4. Seth Gordhandas Sunderdas Medical College and King Edward Memorial Hospital, Mumbai, Maharashtra, India; Journal of Postgraduate Medicine, Seth GS Medical College and KEM Hospital, Mumbai, India, Department of Clinical Pharmacology, Seth GS Medical College and KEM Hospital, Mumbai, India
Chapter keywords

Abstract

A basic measurement of risk is the probability of an individual developing an outcome when exposed to a risk factor. This can be simply expressed as proportion (percentage) of those exposed to the risk factor who develop the outcome, along with its 95% confidence interval. However, to assess the importance of an individual risk factor, it is necessary to compare the risk of the outcome in the exposed group with that in the non-exposed group. A comparison between risks in different groups can be made by examining either their ratio or the difference between them. The 2 × 2 contingency table comes in handy in the calculation of ratios. Odds ratio (OR) is the ratio of the odds of an event in the exposed group, to the odds of the same event in the non-exposed group. It can range from zero to infinity. OR > 1, indicates exposure increases risk while OR < 1 indicates that exposure is protecting against risk. The OR should be presented with its 95% CI to enable more meaningful interpretation—if this interval includes 1, then even a relatively large OR will not carry much weight. The relative risk (RR) denotes ratio of probability of event in exposed group to probability of same event in the non-exposed group. Its interpretation is similar (but not identical) to the OR. If the event in question is relatively uncommon, values of OR and RR tend to be similar. Absolute risk reduction (ARR) is a measure of the effectiveness of an intervention with respect to a dichotomous event. It is calculated as proportion experiencing the event in control group minus the proportion experiencing the event in treated group. It is often used to denote the benefit to the individual. The reciprocal of ARR is the number needed to treat (NNT) and this denotes the number of subjects who would need to be treated in order to obtain one more success than that obtained with a control treatment. Alternatively (e.g. in vaccine trials), this could also denote the number that would need treatment in order to prevent one additional adverse outcome as compared to control treatment. Extended to toxicity, the NNT becomes a measure of harm and is then called number needed to harm (NNH). NNT and NNH are important concepts from the policy maker’s perspective.

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