Week 13 : Common Research Designs in Epidemiology

• Observational Designs for Generating Hypotheses

  • qualitative studies
  • cross - sectional surveys
  • cross - sectional ecologic studies
  • longitudinal ecologic studies
• Research is the process of answering a question that can be answered by appropriate collected data
• Research Question is an interrogative statement that focuses on what variable or concepts are to be described and what relationships might exist between variables
• Research Question is also ;

  • a quantity to estimate
  • a characteristic to describe or compare
  • a relationship to establish
• Research Question : What is (or was) the frequency of a disease in a certain place at a certain time?

  • the answer to this question would be descriptive
  • to answer a question correctly, the data must be obtained and described appropriately
• The rules that governs the process of collecting and arranging the data for analysis are called research designs
• Some research designs are appropriate for hypothesis generation and some are appropriate for hypothesis testing. Some designs can be used for either, depending on the circumstances.
• Research Question : What cause the disease (outcome)?

  Hypothesis Generation - is the process of developing a list of possible candidates for the causes of the disease and obtaining initial evidence that supports one or more of these candidates
• Hypothesis Testing - is done when one or more hypotheses are generated and then tested by making predictions from the hypotheses and examining new data to determine if the predictions are correct
• The basic function of most epidemiologic research designs is either to ;

  • describe the pattern of health and problems accurately or
  • to enable a fair, unbiased comparison to be made beween a group with and a group without a risk factor, a disease, or a preventive or therapeutic intervention
• A good epidemiologic research design should perform the following functions ;

  • enable a comparison of a variable (e.g. disease frequency) between two or more groups at one point in time or, in some cases, within one group before and after receiving an intervention or being exposed to a risk factor
  • allow the comparison to be quantified in absolute terms (as with a risk difference or rate difference) or in relative terms (as with a relative risk or odds ratio)
• A good epidemiologic research design should perform the following functions ;

  • permit the investigators to determine when the risk factor and the disease occured, to determine the temporal sequence
  • minimizes biases, confounding, and other problems that would complicate interpretation of the data
• Types of Research Design

  • some research questions can be answered by more than one type of research design, the choice of design depends on a variety of considerations, including the clinical topic (e.g. whether the disease or conditions is rare or common) and the cost and availability of data
  • research designs are often described as either observational or experimental
  • each type of research design has advantages and disadvantages
• Qualitative Research Advantages

  • generates hypotheses and initial exploration of issues in participants’ own language without bias of investigator
• Qualitative Research Disadvantages

  • cannot test study hypotheses
  • can explore only what is presented or stated
  • has potential for bias
• Cross - Sectional Surveys Advantages

  • are fairly quick and easy to perform
  • are useful for hypothesis generation
• Cross - Sectional Surveys Disadvantages

  • do not offer evidence of a temporal relationship between risk factors and disease
  • are subject to late-looking bias
  • are not good for hypothesis testing
• Ecologic Studies Advantages

  • fairly quick and easy to perform
  • useful for hypothesis generation
• Ecologic Studies Disadvantages

  • do not allow for causal conclusions to be drawn because the data are not associated with individual persons
  • are subject to ecologic fallacy
  • are not good for hypothesis testing
• Cohort Studies Advantages

  • can be performed retrospectively or prospectively
  • can be used to obtain a true (absolute) measure of risk
  • can study many disease outcomes
  • are good for studying rare risk factors
• Cohort Studies Disadvantages

  • are time consuming and costly (especially prospective studies)
  • can study only the risk factors measured at the beggining
  • can be used only for common diseases
  • may have losses to follow up
• Case - Control Studies Advantages

  • fairly quick and easy to perform
  • can study many risk factors
  • good for studying rare diseases
• Case - Control Studies Disadvantages

  • can study many risk factors
  • good for studying rare diseases
  • can obtain only a relative measure of risk
  • subject to recall bias
  • selection of controls may be difficult
  • temporal relationships may be unclear
  • can study only one disease outcome at a time
• Randomized Controlled Trials Advantages

  • gold standard for evaluating treatment interventions (clinical trials) or preventive interventions (field trials)
  • allow investigator to have extensive control over
  • time consuming and usually costly
  • can study only interventions or exposures that are controlled by investigator
  • may have problems related to research process
• Randomized Controlled Trials Disadvantages

  • time consuming and usually costly
  • can study only interventions or exposures that are controlled by investigator
  • may have problems related to therapy changes and dropouts
  • may be limited in generalizability
  • often unethical to perform at all
• Systematic Reviews and Metaanalysis Advantages

  • decrease subjective element of literature review increase statistical power
  • allow exploration of subgroups
  • provide quantitative estimates of effect
• Systematic Reviews and Metaanalysis Disadvantages

  • mixing poor quality studies together in a review or meta-analysis does not improve the underlying quality of studies
• Costeffectiveness Analysis Advantage

  • clinically important
• Costeffectiveness Analysis Disadvantage

  • difficult to identify costs and payments in many health care systems
• Observational Studies

  • investigators observe groups of study participants to learn about the possible effects of a treatment or risk factor
  • assignment of participants to a treatment group or a control group remains outside the investigators control
  • can be either descriptive or analytic
• Experimental Studies

  • the investigators has mere control over the assignment of participants, often placing them in treatment and control groups (e.g. randomization method before the start of any treatment)
• Descriptive

  • distribution of health-related states
  • describes disease occurence as to person, place, time
  • studies generate hypothesis
  • no hypothesis are specified in advance
  • preexisting data are often used
  • associations may or may not be causal
• Analytic

  • determinants of health related states, it examines association between variables
  • studies test hypothesis
  • hypothesis are specified in advance
  • new data are often collected
  • differences between groups are measured
• Qualitative Studies

  • involves an investigation of clinical issues by using anthropologic techniques such as ethnographic observation, open- ended semistructured interviews, focus groups, and key informant interviews
  • investigators attempt to listen to the participants without introducing their own bias as they gather data
  • results are reviewed and patterns in the data are identified in a structured and sometimes quantitative form
• Qualitative Studies

  • results from qualitative research are often invaluable for informing and making a sense of quantitative results and providing greater insights into clinical questions and public health problems
  • the two approaches quantitative and qualitative are complementary with qualitative research providing rich, narrative information that tells a story beyond what reductionist statistics alone might reveal
• Cross - Sectional Surveys

  • aka prevalence survey, a survey of population at a single point in time or over a very short period of time

  may be performed by trained interviewers in ;

  • people’s homes
  • by telephone interviewers
  • by mailed, emailed
  • web-based questionnaires
• Cross - Sectional Surveys

  • useful for determining the prevalence of risk factors and the frequency of prevalent cases of certain diseases for a defined population (estimate the burden of disease)
  • useful for measuring current health status and planning for some health services including setting priorities for disease control (establish baseline data), many surveys have been undertaken to determine the knowledge, attitudes, and health practices or various populations, with the resulting data increasingly being made available to the general public
• Cross - Sectional Surveys Disadvantages

  • data on the exposure to risk factors and the presence or absence of disease are collected simultaneously, creating difficulties in determining the temporal relationship of a presumed cause and effect
  • e.g. difficult to establish the causality (chicken or the egg dilemma)
• Cross - Sectional Surveys Disadvantages

  biased in favor of longer lasting and more indolent (mild) cases of diseases

  • more likely to be found via - survey because people live longer with mild cases, enabling larger numbers of affected people to survive and be interviewed. Severe diseases that tend to be rapidly fatal are less likely to be found
  • neyman bias or late-look bias - it is known as length bias in screening programs, which tend to find (and select for) less agressive illnesses because patients are more likely to be found by screening
  • health - participant bias
• Cross - Sectional Surveys Disadvantages

  • repeated cross - sectional surveys - may be used to determine changes in risk factors and disease frequency in population over time (but not the nature of the association between risk factors and diseases)

  example - participants who reported immunization against a disease had fewer cases of the disease

  • consider if this finding actually meant that people who sought immunization were more concerned about their health and less likely to expose themselves to the disease, known as health-participant bias