Observational study designs

Case report

detailed description of the signs, symptoms, diagnosis, and treatment of an individual patient

Case series

describes a number of patients with a given disease, greater with 1 person

this group of people have common features such as the characteristics of affected patients

case series are oftenly used suggest

emergence of new diseases/ epidemics

previously unknown associations between exposures and outcomes

Pneumocytosis pneumonia

5 young, active, homosexual men were treated for biopsy confirmed pneumocytosis at 3 different hospitals in california. Association suggested bc all patients were homosexuals, must’ve been acquired through sexual contact

limitations of case reports/ case series

impossible to gain strong evidence for causes

• no comparison group for who were not exposed

• esp if assumed cause is common, occurrence of the outcome in people with the exposure may be due to chance

ecological studies

a study in which the units of analysis are populations or groups of people rather than individuals

aggregrate level \

instead of asking each 1st year student we obtain aggregrate data from 20 universities and colleges

an ecologic comparison group

an assessment of the association b/w exposure rates and disease rates during the same time period

why do an ecologic study

individual level data isnt available

often group-level data has already been collected and stored in data archives

If interested in the context of health-how different demographic variables and social environment contribute to mortality or morbidity- ecologic study is a good starting point

Strengths of ecological studies

• Exposuress may truly be population-level 

• Can be easy and inexpensive

  • Often rely on pre-existing data 

• Analysis and presentation are generally simple to understand 

• Can generate new hypotheses for further research

limitations of ecological studies

when a relationship observed at a population level is assumed to occur among individuals 

Potential and alternative explanations cannot be accounted for or tested 

Cross-sectional studies

• Both the exposure and the outcome are determined simultaneously- a snapshot of the study population 

  • Study groups can be based on exposure and the prevalence of outcomes in those exposed and unexposed are compared 

  • Study groups can be based on outcome and the prevalence of the exposure among those with and without the outcome are compared 

• Generally data are collected at an individual level 

Limitations of cross-sectional studies

They yield quantitative estimates of the magnitude of a problem

Don’t measure the temporal ordering of exposure and disease 

Unit of analysis is the person, not a group

strength of cross- sectional studies

helpful in identifying resource needs for health interventions