6: Ecologic, Cross-Sectional, and Case-Control Study Designs

Question 1

Prevalence (point)

Answer 1

all cases of a disease in a population of interest at a specified point in time

Question 2

Incidence

Answer 2

new cases of a disease that occur among a population at risk during a specified period of time

Question 3

How study designs differ (10)

Answer 3

1. number of observations made
2. directionality of exposure
3. data collection methods
4. timing of data collection
5. unit of observation
6. availability of subjects
7. Amount of resources required
8. complexity
9. rigor
10. unit of analysis

Question 4

Manipulation>YES, Randomization>YES, study type?

Answer 4

Experimental

Question 5

Manipulation>YES, Randomization>NO, study type?

Answer 5

Quasi-Experimental

Question 6

Manipulation>NO, Randomization>NO, study type?

Answer 6

Observational

Question 7

Manipulation of study factor

Answer 7

exposure of interest controlled by investigator

Question 8

Randomization of study subjects

Answer 8

random process to determine exposure of study subjects

Question 9

Experimental Studies

Answer 9

maintains greatest control over research

Question 10

Quasi-Experimental Studies

Answer 10

natural experiments (i.e. John Snow)

Question 11

Observational Studies

Answer 11

when it may be impractical or unethical to do experimental

Question 12

Types of Observational Studies

Answer 12

Descriptive and Analytic

Question 13

Descriptive Studies

Answer 13

Cross-sectional and Ecologic (hypothesis generation)

Question 14

Analytic Studies

Answer 14

Case-Control and Cohort (hypothesis testing)

Question 15

2x2 Table

Answer 15

represents association between exposure and disease status

Question 16

Ecologic Studies (unit of analysis? exposure? data type?correlations?)

Answer 16

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

Question 17

Types of Ecologic Studies

Answer 17

Ecologic Comparison and Ecologic Trend

Question 18

Ecologic Comparison Study

Answer 18

examines exposure rates and disease rates among DIFFERENT GROUPS over the same time period. (economic, environmental, lifestyle measures)

Question 19

Ecologic Trend Study

Answer 19

examines CHANGES in exposure and changes in disease within the SAME community, country or other aggregate unit

Question 20

EXAMPLE: The association between breast cancer and dietary fat for 39 countries.

Answer 20

Ecologic Study

Question 21

EXAMPLE: High intakes of dietary fats associated with high rates of breast cancer mortality.

Answer 21

Ecologic Study

Question 22

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?

Answer 22

Ecologic Study

Question 23

EXAMPLE: What are long-term trends (1950-1995) for mortality from the major cancers in the US, Canada, and Mexico?

Answer 23

Ecologic Study

Question 24

EXAMPLE: The effect of fluoridation of the water supply on hip fractures.

Answer 24

Ecologic Study

Question 25

EXAMPLE: The association of naturally occurring fluoride levels and cancer incidence rates.

Answer 25

Ecologic Study

Question 26

EXAMPLE: The relationship between neighborhood or local area social characteristics and health outcomes.

Answer 26

Ecologic Study

Question 27

Ecologic Fallacy

Answer 27

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.)

Question 28

Ecologic Study Advantages

Answer 28

Quick, Simple, Inexpensive, and Good approach for generating hypotheses when a disease is of unknown etiology

Question 29

Ecologic Study Disadvantages

Answer 29

Ecological Fallacy, Imprecise measurement of exposure and disease

Question 30

Cross-Sectional Study (p vs i? unit of analysis? time period? order? sampling used?)

Answer 30

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

Question 31

EXAMPLE: Surveys of smokeless tobacco use among high school students.

Answer 31

Cross-Sectional Study

Question 32

EXAMPLE: Prevalence surveys of the number of vasectomies performed.

Answer 32

Cross-Sectional Study

Question 33

EXAMPLE: Prevalence of congenital malformations across maternal age groups.

Answer 33

Cross-Sectional Study

Question 34

Cross-Sectional Studies Advantages (5)

Answer 34

1. HYPOTHESIS GENERATION 2. INTERVENTION planning 3. PLANNING health services and administering medical care facilities 4. ESTIMATION of the magnitude and distribution of a health problem (quantitative estimate) 5. examine TRENDS in disease or risk factors that can vary over time

Question 35

Cross-Sectional Studies Disadvantages (3)

Answer 35

1. doesn't provide INCIDENCE data (prevalent cases represent survivors) 2. cannot study LOW PREVALENCE diseases. (difficult to sort out risk factors for disease and survival) 3. cannot determine TEMPORALITY of exposure and disease (cause and effect)

Question 36

Case-Control Study

Answer 36

identifies possible causes of disease by finding out how the two groups differ with respect to exposure to some factor

Question 37

Case-Control Characteristics (points of observation? unit of observation, analysis? exposure? data collection? Pvs.I?)

Answer 37

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

Question 38

Sources of Cases (2)

Answer 38

1. identify and enroll all incident cases in a defined population in a specified time period (use disease registries) 2. Medical facilities also may be a source of cases (prevalent) but not always incident cases (represent risk factors? separate causal vs. consequential factors?)

Question 39

Goal of obtaining controls

Answer 39

1. capable of being a case | 2. not necessarily disease free

Question 40

Sources of Controls (3)

Answer 40

1. Population-based (vs. disease registry): ideal scenario, random or obtain from a list 2. Patients from same hospital as cases: live in same environment, so similar exposures. justified when little info is available about exposure-disease association 3. Relatives/Friends of cases: exposures similar but difficult to do because of genetic factors, exposures too similar, not randomly chosen

Question 41

Odds

Answer 41

A/C and B/D

Question 42

Odds Ratio

Answer 42

CASE-CONTROL: AD/BC read as result->exposure

Question 43

OR=1

Answer 43

implies NO association

Question 44

OR=2

Answer 44

suggests cases were twice as likely as controls to be exposed

Question 45

OR<1

Answer 45

suggests a protective factor

Question 46

OR provides a good approximation of risk when: (3)

Answer 46

1. controls are representative of a target population 2. cases are representative of all cases (typical with respect to severity and diagnostic criteria 3. frequency of disease in population is small relative to size of population at risk (rare disease)

Question 47

Case-Control Study Advantages (5)

Answer 47

1. tend to use SMALLER SAMPLE sizes than surveys or prospective studies 2. QUICK and EASY to complete 3. COST EFFECTIVE 4. useful for studies of RARE DISEASES 5. method of choice for INFECTIOUS DISEASE research (identify risk factors with association to disease)

Question 48

Case-Control Study Disadvantages (3)

Answer 48

1. provide INDIRECT estimate of RISK (causal vs. association) 2. unclear TEMPORAL relationships between EXPOSURE and DISEASE (variability of exposure over time) 3. REPRESENTATIVENESS of cases and control often unknown (how were these populations selected?)