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Chapter-08 Measuring disease frequency

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_8
Edition
1/e
Publishing Year
2010
Pages
7
Author Affiliations
1. Seth GS Medical College and Nowrosjee Wadia Maternity Hospital, Mumbai, Maharashtra, India, 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
2. RG Kar Medical College, Kolkata, West Bengal, India, RG Kar Medical College, Kolkata, North Bengal Medical College, West Bengal, India, RG Kar Medical College, Kolkata, India, RG Kar Medical College, Kolkota
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

The key to understanding measures of disease frequency is the three elements of N-E-T—number of persons who are observed for occurrence of the event (i.e. the population), the event (i.e. the disease, condition or outcome of interest) and the time period during which such events are observed. Incidence is the number of new cases occurring in a particular time period. In other words, it measures the rate at which people without a disease develop the same over a specified period of time. Average annual incidence, incidence density, cumulative incidence, attack rate and attributable risk are extensions of this concept. Prevalence is the count of existing disease at a single point in time or over a defined period of time. If over a period of time it will count both new and existing cases. Prevalence may be expressed as point prevalence, period prevalence or cumulative (e.g. lifetime) prevalence. Mortality, incidence and prevalence may be stated as crude or specific rates. The crude rate refers to the number of occurrences for a whole population and is often expressed as rate per 1000, 10,000 or 100,000 members of the population. It may be more meaningful to make rates specific for factors like age, sex, ethnicity and others. Standardized rates adjust for differences in structures between populations. Although age is normally used in this process, other factors (e.g. ethnicity) can also be employed. A single statistic is produced, allowing ready comparisons between populations. Standardization can be done by direct or indirect methods. Both compare a specific study population with a ‘standard population’ (often national population or the standard world population modeled by the World Health Organization). This may be carried out for both sexes individually. Indirect standardization is used more commonly and yields more stable results, than direct standardization, for small populations or small numbers of events. A standardized mortality ratio (SMR) is generated by this method.

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