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What is the purpose of observational studies?

Investigator observes real-life situations and draws influences from them

**subjects are not given any treatment or an exposure


What are the two broad types of observational study designs?

1. groups of populations are compared (Ecological)

2. Groups of individuals are compared (cross sectional, case control, cohort)


T/F: Measurements are made on individuals in ecological studies


**they are all measured at a population level


T/F: Ecological studies are used to test hypotheses



What are the 3 basic steps in ecological studies?

1. Select a sample of populations for inclusion in the study

2. Find population-level statistics on the exposure and outcome for each population

3. Compare groups of populations


How are populations within an ecological study typically grouped?

One group has the exposure, one does not

**remember exposure and or outcome are not measured at the individual level


In an ecological study, there is an association if the amount of disease between two exposure groups is _______


*if both groups have the same rates of illness (or whatever is being measured) there is no association


What are some advantages and disadvantages of ecological studies?

PROS: can be done quickly and inexpensively, analysis and interpretation are relatively simple, can assess a wide range of exposure levels

CONS: Ecological fallacy (relationships observed at the population level may not hold true at the individual level), cannot detect subtle or complicated relationships


Three types of observational/analytical studies based on the reason for selecting study subjects are?

1. To represent a population = Cross sectional

2. Because they have the outcome we are studying = Case Control

3. Because they have the exposure we are studying = Cohort
(these can be prospective or retrospective)


What are the three basic steps in analytical cross sectional studies?

1. Select a sample of subjects from a defined population at one point in time (regardless of their exposure or outcome status)

2. For each individual in the sample - measure BOTH the exposure and outcome (usually at the same time)

3. Compare the groups


When comparing the two groups in an analytical cross sectional study, what measure is used?

Measure of association aka prevalence ration

used to compare the amount of disease in both groups - there is an association if the amount of disease is different between the two exposed groups


What are some advantages and disadvantages to analytical cross sectional studies?

PROS: Can generalize to the population, fast and cheap, provide descriptive or baseline data for future studies

CONS: Not good for causality (can't tell if the dz happened before or after exposure), prevalence is limited in nature


What are case control studies selected to represent?


subjects are selected because of their outcome status (one group will have the outcome, one will not) then the amount of exposure between the groups is compared


In case studies, what population must the subjects choose for case controls come from?

The same population as the outcome cases


Cohort studies select individual subjects to represent what?


subjects are selected by their exposure status - one group will have exposure, one will not

these are used to study the effect of an exposure


What are the two types of cohort studies?

Prospective (forward in time) and retrospective (back in time)


A measure of _____ is used to compare two groups in a cohort study


More specifically: prospective = relative risk

retrospective = prevalence ratios


What are some advantages and disadvantages to prospective cohort studies?

PROS: Better to establish causality, can look at multiple exposures at one time, good for studying rare exposures where it is difficult to get enough statistical analysis

CONS: Can take a long time, have to deal with changing study populations, depends on accurate assessment of outcomes over time


What are some advantages and disadvantages to retrospective cohort studies?

PROS: Short timeline - dz has already occurred, can look at multiple exposures at one time, good for studying rare exposures where it is difficult to get enough subjects for statistical analysis

CONS: not as good at establishing a causality, poor selection of controls can invalidate the entire study, depends upon accurate assessment of disease that happened in the past