non-diseased identified and may quick; (controls) measured they incorporate particularly may be biases; cannot useful for addressed derive rates studying rare Recall bias is a diseases major problem Time-series Large Current (e.g. Current (e.g. Often difficult Many Useful for study community daily) changes daily) to sort out; e.g. confounding studies on (several million in exposure variations in effects of factors; often acute effects people); mortality influenza difficult to susceptible measure groups such as asthmatics Historical Special groups; Records of past Records of past Often difficult Need to rely on Less expensive (retrospective) workers, measurement or current because of records that and quicker cohort study patients, diagnosis retrospective may not be than a insured nature; accurate prospective persons depends on study; can be availability of used to study previously exposures that obtained data no longer exist Prospective Community or Defined at To be Usually easy to Expensive and Can estimate cohort study special groups; outset of study determined measure time incidence and exposed versus (can change during study consuming; relative risk; non-exposed during study) exposure can study categories can many diseases change; high in one study; dropout rate can describe possible associations that suggest cause–effect relationships Experimental Community or Controlled or To be Can be Expensive; Well accepted special groups already known measured controlled by ethical results; strong study during study randomisation considerations; evidence for of subjects study subjects causality or compliance efficacy of required intervention Adapted from: WHO (1991). Epidemiological studies are rarely definitive, and a single epidemiological study cannot establish causality. Any such study is necessarily a sample of the total population of interest, so there will always be questions about being able to generalise from an individual study sample to the total population. Moreover, undetected methodological biases can only be overcome by having numerous studies by different investigators with
non-diseased (controls) identified and measured they may be addressed may incorporate biases; cannot derive rates Recall bias is a major problem quick; particularly useful for studying rare diseases Time-series study Large community (several million people); susceptible groups such as asthmatics Current (e.g. daily) changes in exposure Current (e.g. daily) variations in mortality Often difficult to sort out; e.g. effects of influenza Many confounding factors; often difficult to measure Useful for studies on acute effects Historical (retrospective) cohort study Special groups; workers, patients, insured persons Records of past measurement Records of past or current diagnosis Often difficult because of retrospective nature; depends on availability of previously obtained data Need to rely on records that may not be accurate Less expensive and quicker than a prospective study; can be used to study exposures that no longer exist Prospective cohort study Community or special groups; exposed versus non-exposed Defined at outset of study (can change during study) To be determined during study Usually easy to measure Expensive and time consuming; exposure categories can change; high dropout rate possible Can estimate incidence and relative risk; can study many diseases in one study; can describe associations that suggest cause–effect relationships Experimental study Community or special groups Controlled or already known To be measured during study Can be controlled by randomisation of subjects Expensive; ethical considerations; study subjects compliance required Well accepted results; strong evidence for causality or efficacy of intervention Adapted from: WHO (1991). Epidemiological studies are rarely definitive, and a single epidemiological study cannot establish causality. Any such study is necessarily a sample of the total population of interest, so there will always be questions about being able to generalise from an individual study sample to the total population. Moreover, undetected methodological biases can only be overcome by having numerous studies by different investigators with
w.Environmental-Health-Risk-Assessment.docx