6: Ecologic, Cross-Sectional, and Case-Control Study Designs Flashcards
Prevalence (point)
all cases of a disease in a population of interest at a specified point in time
Incidence
new cases of a disease that occur among a population at risk during a specified period of time
How study designs differ (10)
- number of observations made
- directionality of exposure
- data collection methods
- timing of data collection
- unit of observation
- availability of subjects
- Amount of resources required
- complexity
- rigor
- unit of analysis
Manipulation>YES, Randomization>YES, study type?
Experimental
Manipulation>YES, Randomization>NO, study type?
Quasi-Experimental
Manipulation>NO, Randomization>NO, study type?
Observational
Manipulation of study factor
exposure of interest controlled by investigator
Randomization of study subjects
random process to determine exposure of study subjects
Experimental Studies
maintains greatest control over research
Quasi-Experimental Studies
natural experiments (i.e. John Snow)
Observational Studies
when it may be impractical or unethical to do experimental
Types of Observational Studies
Descriptive and Analytic
Descriptive Studies
Cross-sectional and Ecologic (hypothesis generation)
Analytic Studies
Case-Control and Cohort (hypothesis testing)
2x2 Table
represents association between exposure and disease status
Ecologic Studies (unit of analysis? exposure? data type?correlations?)
Unit of Analysis: group, NOT individual
Level of Exposure: unknown for each individual in unit being studied
Data Type: generally secondary data
Correlations: obtained between exposure rate and disease rates among different groups or populations
Types of Ecologic Studies
Ecologic Comparison and Ecologic Trend
Ecologic Comparison Study
examines exposure rates and disease rates among DIFFERENT GROUPS over the same time period. (economic, environmental, lifestyle measures)
Ecologic Trend Study
examines CHANGES in exposure and changes in disease within the SAME community, country or other aggregate unit
EXAMPLE: The association between breast cancer and dietary fat for 39 countries.
Ecologic Study
EXAMPLE: High intakes of dietary fats associated with high rates of breast cancer mortality.
Ecologic Study
EXAMPLE: Is the ranking of cities by air pollution levels associated with the ranking of cities by mortality from CVD, adjusting for differences in average age, percent of the population below poverty level, and occupational structure?
Ecologic Study
EXAMPLE: What are long-term trends (1950-1995) for mortality from the major cancers in the US, Canada, and Mexico?
Ecologic Study
EXAMPLE: The effect of fluoridation of the water supply on hip fractures.
Ecologic Study
EXAMPLE: The association of naturally occurring fluoride levels and cancer incidence rates.
Ecologic Study
EXAMPLE: The relationship between neighborhood or local area social characteristics and health outcomes.
Ecologic Study
Ecologic Fallacy
Observations made at the group level may not represent the exposure-disease relationship at the individual level. Occurs when incorrect inferences about the individual are made from group data. The conclusions obtained from an ecologic study may be the reverse of those from a study that collects data on individual subjects (sunburn vs. hat ex.)
Ecologic Study Advantages
Quick, Simple, Inexpensive, and Good approach for generating hypotheses when a disease is of unknown etiology
Ecologic Study Disadvantages
Ecological Fallacy, Imprecise measurement of exposure and disease
Cross-Sectional Study (p vs i? unit of analysis? time period? order? sampling used?)
Pvs.I: type of prevalence study
Unit of Analysis: exposure and disease measures obtained at the INDIVIDUAL level
Time Period: single period of observation
Order: select sample of subjects>then determine exposure and disease histories collected simultaneously
Sampling Used: both probability and non-probability
EXAMPLE: Surveys of smokeless tobacco use among high school students.
Cross-Sectional Study
EXAMPLE: Prevalence surveys of the number of vasectomies performed.
Cross-Sectional Study
EXAMPLE: Prevalence of congenital malformations across maternal age groups.
Cross-Sectional Study
Cross-Sectional Studies Advantages (5)
- HYPOTHESIS GENERATION
- INTERVENTION planning
- PLANNING health services and administering medical care facilities
- ESTIMATION of the magnitude and distribution of a health problem (quantitative estimate)
- examine TRENDS in disease or risk factors that can vary over time
Cross-Sectional Studies Disadvantages (3)
- doesn’t provide INCIDENCE data (prevalent cases represent survivors)
- cannot study LOW PREVALENCE diseases. (difficult to sort out risk factors for disease and survival)
- cannot determine TEMPORALITY of exposure and disease (cause and effect)
Case-Control Study
identifies possible causes of disease by finding out how the two groups differ with respect to exposure to some factor
Case-Control Characteristics (points of observation? unit of observation, analysis? exposure? data collection? Pvs.I?)
Points of observation: single point (disease or not)
Unit of observation: individual
Unit of analysis: individual
Exposure: determined retrospectively
Data Collection: typically combination of primary and secondary sources
Pvs.I: doesn’t directly provide incidence data
Sources of Cases (2)
- identify and enroll all incident cases in a defined population in a specified time period (use disease registries)
- Medical facilities also may be a source of cases (prevalent) but not always incident cases (represent risk factors? separate causal vs. consequential factors?)
Goal of obtaining controls
- capable of being a case
2. not necessarily disease free
Sources of Controls (3)
- Population-based (vs. disease registry): ideal scenario, random or obtain from a list
- Patients from same hospital as cases: live in same environment, so similar exposures. justified when little info is available about exposure-disease association
- Relatives/Friends of cases: exposures similar but difficult to do because of genetic factors, exposures too similar, not randomly chosen
Odds
A/C and B/D
Odds Ratio
CASE-CONTROL: AD/BC read as result->exposure
OR=1
implies NO association
OR=2
suggests cases were twice as likely as controls to be exposed
OR<1
suggests a protective factor
OR provides a good approximation of risk when: (3)
- controls are representative of a target population
- cases are representative of all cases (typical with respect to severity and diagnostic criteria
- frequency of disease in population is small relative to size of population at risk (rare disease)
Case-Control Study Advantages (5)
- tend to use SMALLER SAMPLE sizes than surveys or prospective studies
- QUICK and EASY to complete
- COST EFFECTIVE
- useful for studies of RARE DISEASES
- method of choice for INFECTIOUS DISEASE research (identify risk factors with association to disease)
Case-Control Study Disadvantages (3)
- provide INDIRECT estimate of RISK (causal vs. association)
- unclear TEMPORAL relationships between EXPOSURE and DISEASE (variability of exposure over time)
- REPRESENTATIVENESS of cases and control often unknown (how were these populations selected?)
