Cross Sectional & Cohort Studies

Question 1

observational studies

Answer 1

investigator observes study participants (does not assign exposure) and records exposure and outcome

investigator can NOT assign exposure but CAN select subjects accordingly

Question 2

do observational studies use randomization

Answer 2

NO - randomization requires intervention (clinical trials only)

CAN do random selection of subjects from the population

Question 3

types of observational studies

Answer 3

1. cross sectional
2. cohort
3. case control
4. clinical case series

Question 4

cross sectional studies

Answer 4

study an entire population at a single point in time

sample: whole population OR random sample independent of exposure and disease

cases: prevalence NOT incidence because not evaluating over time

Question 5

prospective cross sectional studies

Answer 5

1. random group selected
2. test for disease
3. determine if they have exposure

Question 6

retrospective cross sectional studies

Answer 6

1. random group selected
2. were they + or - for disease
3. were they exposed

Question 7

advantages of CS studies

Answer 7

• generates hypothesis for clinical trials
• quick, easy, low cost
• can include more than 1 exposure and disease
• represents an entire population

Question 8

disadvantages of CS studies

Answer 8

• temporality: can not confirm order of exposure and disease
• causality: can not determine causality of exposure on disease because they are non-directional
• survivor bias: only includes living subjects
• misleading in short duration and chronic diseases

Question 9

cohort studies

Answer 9

study a single cohort of subjects over time and record disease at it occurs

randomly selected subjects that all have something in common at enrollment

Question 10

steps of cohort studies

Answer 10

1. randomly select disease free subjects
2. divide into exposed and not exposed
3. follow each group and determine who develops disease

Question 11

what analysis is made from cohort studies

Answer 11

disease incidence (risk) in exposed and unexposed subjects

if incidence is higher in exposed than unexposed, the exposure MAY be causal to outcome

Question 12

are there confounders in observational studies

Answer 12

YES - can not ID every confounder because there is no randomization

Question 13

prospective cohort studies

Answer 13

disease free subjects are divided into exposed and not exposed then followed over time to see if disease develops

can confirm temporality (exposure came before disease) because enrollment is disease free subjects

Question 14

retrospective cohort studies

Answer 14

disease free subjects are ID’d from records then divided into exposed and not exposed

can NOT confirm temporality

Question 15

advantages of cohort studies

Answer 15

• best observational studies for COMMON diseases
• works well with rare EXPOSURES only
• no selection bias because all are disease negative at enrollment

Question 16

disadvantages of cohort studies

Answer 16

• does NOT work well for RARE diseases
• can be costly or difficult if studying long term diseases
• loss of follow up can be a problem

Question 17

what can be calculated from a cohort study

Answer 17

incidence proportion
incidence rate
hazard ratio
risk ratio
incidence rate ratio

Question 18

incidence proportion

Answer 18

risk of developing a disease in a population

newly diseased over time / entire population

Question 19

incidence rate

Answer 19

risk of developing a disease over a period of time

newly diseased over time / sum of days that individual subjects are at risk

Question 20

risk ratio

Answer 20

the ratio of disease incidence proportion in the exposed to that in the non exposed

Question 21

incidence rate ratio

Answer 21

the ratio of disease incidence rate in the exposed to that in the unexposed

Question 22

hazard ratio

Answer 22

the ratio of hazard of disease in exposed to that in the unexposed