Cumulative exposure index. It includes time in its units. Mean grade. The mean grade MG cumulates the products of exposure grade and exposure duration for each successive job title i. MG is independent of time in its units; the summary measure for a person exposed for a long period at a high concentration will be similar to that for a person exposed for a short period at a high concentration. Within any matched set in a case-control design, MG is an average grade of exposure per unit of time exposed.
It is an average grade for the time actually exposed to the agent under consideration. Highest grade ever. The highest grade ever HG is determined from scanning the work history for the highest grade assignment in the period of observation to which the worker was exposed for at least seven days.
Time-weighted average grade. Within any matched set in a case-control design, the TWA grade averages over total time employed. It differs from MG, which averages only over the total time actually exposed. Thus, TWA grade can be viewed as an average exposure per unit of time in the full term of employment regardless of exposure per se. Total time exposed. The total time exposed TTE accumulates all time periods associated with exposure in units of time.
TTE has appeal for its simplicity. However, it is well accepted that health effects must be related not only to duration of chemical exposure, but also to the intensity of that exposure i. Clearly, the utility of a summary exposure measure is determined by the respective weight it attributes to either duration or concentration of exposure or both. Thus different measures may produce different results Walker and Blettner Ideally, the summary measure selected should be based on a set of defensible assumptions regarding the postulated biological mechanism for the agent or disease association under study Smith This procedure is not, however, always possible.
Very often, the biological effect of the duration of exposure or the concentration of the agent under study is unknown. In this context, the use of different exposure measures may be useful to suggest a mechanism by which exposure exerts its effect. It is recommended that, in the absence of proved models for assessing exposure, a variety of summary worklife exposure measures be used to estimate risk.
This approach would facilitate the comparison of findings across studies. Epidemiology involves measuring the occurrence of disease and quantifying associations between diseases and exposures. Disease occurrence can be measured by frequencies counts but is better described by rates, which are composed of three elements: the number of people affected numerator , the number of people in the source or base population i.
Statistical Models in Epidemiology, the Environment, and Clinical Trials
The denominator of the rate is the total person-time experienced by the source population. Rates allow more informative comparisons between populations of different sizes than counts alone. Risk, the probability of an individual developing disease within a specified time period, is a proportion, ranging from 0 to 1, and is not a rate per se. Attack rate, the proportion of people in a population who are affected within a specified time period, is technically a measure of risk, not a rate.
Disease-specific morbidity includes incidence, which refers to the number of persons who are newly diagnosed with the disease of interest. Prevalence refers to the number of existing cases. Mortality refers to the number of persons who die.
Course BIOS Statistical Models for Epidemiology
Incidence is defined as the number of newly diagnosed cases within a specified time period, whereas the incidence rate is this number divided by the total person-time experienced by the source population table For cancer, rates are usually expressed as annual rates per , people. Rates for other more common diseases may be expressed per a smaller number of people. For example, birth defect rates are usually expressed per 1, live births.
Cumulative incidence, the proportion of people who become cases within a specified time period, is a measure of average risk for a population. Point prevalence at end of year 5. Annual mortality rate. Prevalence includes point prevalence, the number of cases of disease at a point in time, and period prevalence, the total number of cases of a disease known to have existed at some time during a specified period. Mortality, which concerns deaths rather than newly diagnosed cases of disease, reflects factors that cause disease as well as factors related to the quality of medical care, such as screening, access to medical care, and availability of effective treatments.
Consequently, hypothesis-generating efforts and aetiological research may be more informative and easier to interpret when based on incidence rather than on mortality data. However, mortality data are often more readily available on large populations than incidence data. The term death rate is generally accepted to mean the rate for deaths from all causes combined, whereas mortality rate is the rate of death from one specific cause.
For a given disease, the case-fatality rate technically a proportion, not a rate is the number of persons dying from the disease during a specified time period divided by the number of persons with the disease. The complement of the case-fatality rate is the survival rate. The five-year survival rate is a common benchmark for chronic diseases such as cancer.
The occurrence of a disease may vary across subgroups of the population or over time. A disease measure for an entire population, without consideration of any subgroups, is called a crude rate. For example, an incidence rate for all age groups combined is a crude rate. The rates for the individual age groups are the age-specific rates. To compare two or more populations with different age distributions, age-adjusted or, age-standardized rates should be calculated for each population by multiplying each age-specific rate by the per cent of the standard population e.
Rates can be adjusted for factors other than age, such as race, gender or smoking status, if the category-specific rates are known. Surveillance and evaluation of descriptive data can provide clues to disease aetiology, identify high-risk subgroups that may be suitable for intervention or screening programmes, and provide data on the effectiveness of such programmes.
Sources of information that have been used for surveillance activities include death certificates, medical records, cancer registries, other disease registries e. Epidemiology attempts to identify and quantify factors that influence disease.