Public Health Surveillance Study Designs

Examples of intervention and outcomes

• Measles vaccine (intervention) and measles disease (outcome)

• Hand hygiene messaging (intervention) and improved handwashing (outcome)

Examples of exposures and outcomes

• smoking (exposure) and lung cancer (outcome)

• potato salad (exposure) and gastrointestinal illness (outcome)

• BMI (risk factor) and diabetes (outcome)

Confounding

• complex relationships among variables that can distort relationships between risk factors and disease

• need to remove the effect of confounding before we can describe the relationship

Information bias

incorrect measurement or classification of outcome (or exposure) status

• recall bias

Selection bias

the prevalence of exposure and disease may be different in those who chose to participate as opposed to those who did not

• non-response bias

• loss to follow-up

study design

methodology used to collect the information to address the research question

observational studies

• we observe a phenomenon

• sometimes called nonrandomized, or historical studies

Experimental studies

• we intervene and measure a response

• sometimes called randomized or clinical trials

types of observational study designs

• case report/case series

• cross-sectional survey

• cohort study

• case-control study

Case report:

Detailed report of specific features of case (1case only)

Case series:

systematic review of common features of a small number of cases (5 cases)

• advantages: cost-efficient, easy to conduct

• disadvantages: no comparison group, no specific research question

Cross-sectional surveys

• conducted at one point in time

• advantages: cost-efficient, easy to implement, ethical

• disadvantages: no temporal information

when are cross-sectional surveys used

• when measuring prevalence of disease or practice

• information for practical decision making

Case-control studies

a study involving individuals with (cases) and without (controls) outcome of interest

• advantages: cost and time efficient for rare outcomes

• disadvantages: need careful selection of cases and controls, bias

when are case-control studies used

• when the outcome is rare

• limited resources

• when you are not interested in incidence rates

cohort studies

• a study involving a group (cohort) of individuals who meet inclusion criteria at the start of a study

Prospective cohort study (concurrent; longitudinal)

individuals are enrolled and followed going forward in time

Retrospective Cohort Study (Non-concurrent; historical)

The exposure or risk factor is ascertained by looking back in time (or retrospectively)

advantages of a cohort study

assess temporal relationships

• estimate and compare incidence of disease

  • the rate at which participants free of the disease develop the disease

disadvantages of a cohort study

• need large numbers for rare outcomes

• confounding (distortion of the effect of a risk factor on the outcome by other characteristics)

when are cohort studies used

• when determining the incidence of an outcome

• exposure/risk factor is rare

• when you have a lot of time and resources

Ex: Framingham cardiovascular studies, nurses health study

Experimental Study Designs

• Randomized controlled trial (“clinical trial)

• crossover trial

Randomized controlled trials

• Experimental study where patients are randomized to receive one of several comparison treatments

advantages to a randomized control trial

• gold standard from a statistical point of view

• casual inference

• minimizes bias and confounding

disadvantages of a randomized control trial

• expensive

• requires extensive monitoring

• inclusion criteria can limit generalizability

when are randomized controlled trials used

• when you want to find efficacy of intervention

• when feasible and ethical to randomize

Ex: studies of investigational drugs or vaccines, studies of non-pharmaceutical interventions or programs

crossover trial

each participant is assigned to two or more treatments sequentially

• washout period in between treatments

• advantages: each participant is their own control

• disadvantages: carry over effects

what is the key difference between an observational and experimental study

in an experimental study we impose a change to their lives (intervention) while in an observational study we just observe