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Presentation to the Module I: Clinical Epidemiology at a Clinical Research Coordinator Course held on 5-9 January 2020 at Faculty of Medicine, King Fahad Medical City, Riyadh. by Professor Omar Hasan Kasule Sr. MB ChB (MUK), MPH (Harvard), DrPH (Harvard), Chairman of the KFMC IRB
LEARNING OBJECTIVES:
• Definitions of rates, hazards, ratios, and proportions.
• Crude rates, specific rates, and standardized/adjusted rates.
KEYWORDS AND TERMS:
• Denominator and numerator
• Parameter and statistic
• Proportion
• Rate: crude, specific, adjusted rate, standardized rate, crude rate
• Rate, hazard rate, the incidence rate
• Ratio, odds ratio, risk ratio
DEFINITIONS:
• The main types of statistics used are measures of location such as rates, hazards, ratios, and proportions, and measures of spread.
• Measures of location indicate accuracy or validity.
• Measures of spread or variation, such as variance and range, indicate precision.
RATES:
• A rate is the number of events in a given population over a defined time period and has 3 components: a numerator, a denominator, and time. The numerator is included in the denominator.
• The incidence rate of disease is defined as a /{(a+b)t} where a = number of new cases, b = number free of disease at the start of time interval, and t = duration of the time of observation.
RATES: Types
• A crude rate for a population assumes homogeneity and ignores subgroups' differences. It is therefore un-weighted, misleading, and unrepresentative. Inference and population comparisons based on crude rates are not valid.
• Rates can be specific for age, gender, race, and cause. Specific rates are more informative than crude rates but are cognitively difficult to internalize, digest, and understand so many rates or be able to reach some conclusions.
• An Adjusted or standardized rate is a representative summary that is a weighted average of specific rates free of the deficiencies of both the crude and specific rates. Standardization eliminates the ‘confusing’ or ‘confounding’ effects due to subgroups.
• Life expectancy is a form of age-standardized standardized mortality rate. Regression techniques provide a means of simultaneous adjustment of the impact of various factors on the rate.
HAZARDS:
• A hazard is defined as the number of events at time t among those who survive until time t.
• Hazard can also be defined as a relative hazard with respect to a specific risk factor.
• At a specific point in time, relative hazard expresses the hazard among the exposed compared to the hazard among the non-exposed.
RATIOS:
• Ratio is generally defined as a: b where a= number of cases of disease and b = number without the disease.
• Examples of ratios are the proportional mortality ratio, the maternal mortality ratio, and the fetal death ratio.
• The proportional mortality ratio is the number of deaths in a year due to a specific disease divided by the total number of deaths in that year. This ratio is useful in occupational studies because it provides information on the relative importance of a specific cause of death.
• The maternal mortality ratio is the total number of maternal deaths divided by the total live births. The fetal death ratio is the ratio of fetal deaths to live births.
PROPORTIONS:
• A proportion is the number of events expressed as a fraction of the total population at risk without a time dimension.
• The formula of a proportion is a/(a+b) and the numerator is part of the denominator.
• Examples of proportions are prevalence proportion, neonatal mortality proportion, and the perinatal mortality proportion.
• The term prevalence rate is a common misnomer since prevalence is a proportion and not a rate. Prevalence describes a still/stationary picture of the disease.
• Like rates, proportions can be crude, specific, and standard. The variance of a sample proportion can be computed as {p(1-p)/n} if n < 0.05 or N = ¥ or N is large in relation to n.