Intro To Epidemiology - Lectures 5-7

Front 1

What is a p-value?

Back 1

The probability of obtaining results as or more extreme, by chance alone, given that H-null is true.

Front 2

What is the interpretation of a confidence interval?

Back 2

"The interval was produced by a method that contains the true value 95% of the time in the absence of bias."

Front 3

What is the source population?

Back 3

The population from which the study population and base is selected by use of exclusions.

(the pool from which we're pulling people)

Front 4

What is the study population?

Back 4

The population whose experience constitutes the study base.

(people we select)

Front 5

What is the study base?

Back 5

The population experience captured in a study.

(experiences we select to study)

Front 6

What is a confounder?

Back 6

An extraneous determinant of the outcome that biases the association.

Front 7

What is a study?

Back 7

A project designed to yield evidence for the advancement of knowledge.

Front 8

What are the two overarching categories of epidemiologic study designs?

Back 8

Experimental and observational

Front 9

What is an experimental study design?

Back 9

One in which the determinant/exposure is intentionally assigned solely for the goals of the study. (randomized controlled trial)

Front 10

What is an observational study design?

Back 10

One in which the determinant/exposure is determined by some other mechanism (non-assigned).

-cohort
-case-control
-cross-sectional
-Ecologic

Front 11

What is the study base of a cross-sectional study?

Back 11

A population cross-section (as opposed to a population experience over time)

Front 12

When is the outcome assessed in a cross-sectional study?

Back 12

At a SINGLE point in time. (A study of PREVALENT cases)

Determinant usually assessed at a single point in time.

Front 13

What can be uncertain about cross-sectional studies?

Back 13

Temporality of determinant and outcome.

(Which came first - determinant or outcome?)

Front 14

What does a cross-sectional study allow for the estimation of?

Back 14

The prevalence ratio and prevalence difference.

Front 15

Is there follow-up in a cross-sectional study?

Back 15

No.

Front 16

Which measures of association can be estimated in a cross-sectional study?

Back 16

Prevalence rate or prevalence difference.

Front 17

What are the strengths of a cross-sectional study? (there are 2)

Back 17

-Relatively quick and inexpensive

- Good for early stages of knowledge

Front 18

What are the limitations of a cross-sectional study? (there are 2)

Back 18

-Can have unclear temporality of determinant and outcome

-Since cross-sectional studies use prevalent cases, the duration of illness can bias estimated association if the determinant affects the duration of illness.

Front 19

What is the key feature of an ecologic study?

Back 19

It is a study of the determinant and the outcome at the population level, and NOT an individual level.

Front 20

What type of bias is an ecologic study subject to?

Back 20

Ecologic fallacy!

Front 21

How would the study below be changed to an ecologic study?

Does smoking cause death from coronary disease?

Back 21

Do states with higher smoking levels have higher coronary disease mortality rates than states with lower smoking levels?

Front 22

What is population level information?

Back 22

Each observation represents a group (e.g. countries, cities, year) rather than an individual.
Ex: Particulate concentration in air OR asthma hospitalization rate

Front 23

What would the determinant and outcome be for an ecologic study asking:
Do states with higher smoking levels have higher coronary disease mortality rates than states with lower smoking levels?

Back 23

Determinant: Per capita cigarette sales

Outcome: heart disease mortality rates

Front 24

What is ecologic fallacy bias?

Back 24

Estimates of an effect in an ecologic study does not reflect effects at the individual level.

-People with outcome may not be those with the determinant of interest.
(Interest in individual-level effects but measuring with group-level data)

Front 25

What are the strengths of ecologic studies? (there are 4)

Back 25

-They are inexpensive
-Good for early stages of knowledge
-May allow for a wider range of exposures
-Can help study ecologic relationships (e.g. effect of seat belt laws on motor vehicle fatalities)

Front 26

What measures of association are used for ecologic studies?

Back 26

linear regression to estimate the slope of the regression line beta.

(How much Y changes for a one-unit change in X)

Front 27

What are the limitations of an ecologic study?

Back 27

-Ecoligic bias (group data to study individual-level associations
-Little to no control of confounding

Front 28

What is the study base of a cohort study?

Back 28

The population experience over time

Front 29

What are the defining features of a cohort study?

Back 29

A cohort study involves follow-up (documentation) of cohort members over time.

-It's a study of incidence (e.g. diagnoses) or change (e.g. progression or improvement of illness)

Front 30

What is estimated for a cohort study?

Back 30

risk and rates

Front 31

What are the key features of a randomized controlled trial?

Back 31

-prognistic study about the future course of health or illness
-study of INCIDENCE (or change)
-An experiment; the determinant/exposure is intentionally assigned for the goals of the study

Front 32

In an RCT, what forms the study population and study base?

Back 32

Enrollment into the cohort.

Involves follow-up of cohort members.

Front 33

What is the source population in an RCT?

Back 33

individuals with indication and without contraindication for intervention enrolled in the cohort

Front 34

What is the study population in an RCT?

Back 34

Formed by enrollment into the cohort

Front 35

What is the study base in an RCT?

Back 35

The follow-up experience of individuals in the cohort

Front 36

What are two different types of RCTs?

Back 36

Treatment allocated to INDIVIDUALS

Treatment allocated to ENTIRE COMMUNITIES (number of communities, not individuals, constitutes the study size

Front 37

What are the two ways to minimize systematic study error (bias) in a RCT?

Back 37

-Use of a placebo (allow study to be "blinded", make groups as comparable as possible)

-Maintaining high compliance (run-in period, compensation, frequent contact)

Front 38

what is a run-in period for an RCT?

Back 38

Certain number of weeks on treatment to find compliant subjects. Only use compliant subjects (randomize after suitable amount of time)

Front 39

What measures of association are used for RCTs?

Back 39

-Risk ratio:
(often presented as excess risk: (1-RR)*100%
or risk difference
-also can estimate rate ratio and rate difference

Front 40

What is intent to treat analysis?

Back 40

Includes all participants based on their treatment assignment; non-compliers are considered having complied with assigned treatment

Front 41

what are the advantages of intent-to-treat analysis?

Back 41

Maintains randomization, groups are balanced

Front 42

What are the disadvantages of intent-to-treat analysis?

Back 42

May obscure real therapeutic effect among those who actually had the intervention

Front 43

What is Efficacy analysis (aka as-treated analysis)?

Back 43

Includes only participants who received the treatment as assigned (only compliers)

Front 44

what are the advantages of efficacy analysis?

Back 44

Tests effect of treatment in those who were actually treated

Front 45

What are the disadvantages of efficacy analysis?

Back 45

Lose randomization, so groups may not be balanced

Front 46

What are the 2 main strengths of randomized controlled trials?

Back 46

1. Can minimize bias

2. They are optimal to detect statistically small to moderate (but clinically worthwhile) effects

Front 47

What are the 3 main limitations of randomized controlled trials?

Back 47

1. Acceptability (??)
2. Feasibility: time, cost
3. Equipoise: (humanitarian viewpoint) have to have enough belief to give treatment, enough doubt to withhold treatment

Front 48

When one initiates a cohort, what does it involve?

Back 48

Follow-up of people, data collection, etc. for the occurrence of illness (or change)

Front 49

For a cohort study, how is membership defined?

Back 49

Membership into the cohort defined on the basis of some event

Person is forever after a cohort member

Front 50

What types of studies can be conducted in a cohort?

Back 50

Cohort study designs
cross-sectional study designs
case-control study designs

Front 51

What are the different types of cohort studies?

Back 51

-non-experimental prognostic study
-etiologic study
-descriptive study

Front 52

What is studied/estimated in a cohort study?

Back 52

-Incidence (or change) studied
-Risks or rates estimated

-involves follow-up of cohort members

Front 53

What is the key feature of a non-experimental cohort study as opposed to an RCT?

Back 53

The determinant/exposure is NOT assigned by the investigator

Front 54

How is the outcome assessed in a cohort study?

Back 54

Many different ways (examples below):

-Medical or occupational records
-Routine examinations at cohort visits (lab tests, physical measurements)
-Questionnaires

Front 55

How is the determinant assessed in a cohort study?

Back 55

Lots of different ways, just like outcome:

-Medical or occupational records
-Routine examinations at cohort visits (lab tests, physical measurements)
-Questionnaires

Determinant may be fixed (blood type) or may change over time (smoking)

Front 56

What is the outcome of a cohort study?

Back 56

Incidence (e.g. diagnosis)
OR
change (e.g. progression or improvement of illness)

Front 57

What are the two ways a reference group can be chosen for a cohort study?

Back 57

Internal or External reference group

Front 58

What is an internal reference group for a cohort study?

Back 58

Members of the same cohort WITHOUT the characteristic of interest

Front 59

What is an external reference group for a cohort study?

Back 59

Members of another cohort WITHOUT the characteristic of interest
OR
subjects from the general population

Front 60

What are the strengths of using an internal reference group?

Back 60

It's the best reference group when feasible

Comes closest to the "counterfactual ideal" because it is most comparable to the index (exposed) group

Front 61

Why would an external reference of another cohort group be used?

Back 61

-Often used in studies of occupational exposures --> Another occupational cohort could share some behavioral risk factors

-A working reference cohort eliminates the Healthy Worker Effect

Front 62

What is one caution about using an external reference group of another cohort?

Back 62

-Need to be careful that the reference cohort doesn't have its own characteristics that may affect illness

Front 63

Why would an external reference group of the general population be used?

Back 63

-It's accessible and cheaper

Front 64

What are some cautions about using an external reference group of the general population?

Back 64

-limited on outcomes with available information
-Overall population includes individuals with and without the determinant of interest
-Healthy worker effect
-Limited data on confounding variables

Front 65

What measures of association are used for a cohort study?

Back 65

-rate ratio and rate difference

(Can validly estimate risk ratio or risk difference when most individuals are followed for the entire risk period)

Front 66

What is non-differential loss to follow-up and what does it affect?

Back 66

Non-differential = not related to outcome or determinant

Affects study size

Front 67

What is differential loss to follow-up and what does it affect?

Back 67

Differential = related to both outcome and determinant

Affects study validity (selection bias)

Front 68

How can we maintain follow-up (study retention)?

Back 68

-Make it easy for the subject to participate and update you
-Incentives
-Maintain contact with the subject (study visits, questionnaires, study updates, cards, keep notes on contact, update addresses, etc)

Front 69

What is a prospective cohort study?

Back 69

A study that starts prior to any illness occurrence in the entire cohort

Front 70

What is a retrospective cohort study?

Back 70

A study that starts after all illness in the entire cohort has occurred.

Front 71

What are the positives of a retrospective study?

Back 71

-Time/costs
-Better for diseases with long illness induction

Front 72

What are the positives of a prospective study?

Back 72

-Data quality on determinant
-Data quality on confounders

Front 73

What are the strengths of cohort studies?

Back 73

-Efficient for rare exposures
-Collection of data in a cohort allows evaluation of determinant in relation to many illnesses
-Temporality of determinant-illness usually clear
-Determinant history documented before the illness occurs

Front 74

What are the limitations of cohort studies?

Back 74

-Costly for rare outcomes or illnesses with long induction periods
-Expensive to follow individuals, collect data, conduct routine medical examinations and lab tests etc
-can have loss to follow-up

Front 75

What are the key features of a case-control study?

Back 75

-Includes cases in the study base and a SAMPLE of the population experience of the study base (controls)
-Can have a primary or secondary study base
-Usually ETIOLOGIC study
-Usually a study of incidence

Front 76

What is estimated in a case-control study?

Back 76

Incidence odds ratio

Front 77

Why conduct a case-control study?

Back 77

-Efficient design to study an etiologic question, especially when:
1.Illness is rare
2. Exposure is difficult or expensive to obtain
3. Disease has a long time between exposure and disease
4. Difficult to follow and track individuals in a particular dynamic population

Front 78

In a cohort study, we have complete documentation of the experience of the entire study base (i.e. follow-up). What do we have in a case-control study?

Back 78

Cases (individuals with the outcome) and a SAMPLE of the experience of the study base (controls)

We sample for the control

Front 79

What does a 2x2 table look like for a CASE-CONTROL study?

Back 79

Columns = determinant (index on L, Reference on R)
Rows = Cases (top) and Controls (bottom)

Front 80

What is a primary study base?

Back 80

-The source population that gave rise to the cases is KNOWN FIRST and THEN cases are identified

(Cases come from a population that is explicitly and directly defined)

The study base is defined directly (and not indirectly through case identification)

Front 81

Where do the controls come from in a primary study base?

Back 81

Examples:
-Cohort (in a case-control study conducted within a cohort/fixed population)
-National identity registries
-Town lists
-Random digit-dialed telephone numbers

Front 82

How is a secondary study base defined?

Back 82

-Cases come from a population that cannot be explicitly defined
(We identify cases first then try to identify the source population that gave rise to the cases)

-The study base is INDIRECTLY defined; it is defined secondarily to the identification of the cases

Front 83

When do conditions arise for a secondary study base?

Back 83

Examples:
-When cases are identified from hospitals whose catchment areas cannot be specified
-From other medical practices serving patients with various places of residence, health insurance, referral patterns
-Disease registry that does not cover a defined population

Front 84

Where do the controls come from ion a secondary study base?

Back 84

Examples:
-Family, friends
-Residents of the same neighborhood
-Patients admitted to the hospital with other illness unrelated to the determinant of interest
-Patients attending the clinic for other illness unrelated to determinant of interest

Front 85

What are the potential strengths of using a primary study base?

Back 85

-It's clear that controls represent the study base (population experience) that produced the cases (same source population)

-In a case-control study conducted in a cohort: data and specimens often already drawn and stored (quick and inexpensive to conduct as long as specimen quality not an issue)

Front 86

What are some limitations to using a primary study base in a case-control study?

Back 86

-Expensive to obtain controls: random digit dialing
-Controls tend to be healthy so:
a. They can be less interested in participating and lower response rate (selection bias)
b. Accuracy of determinant information may be different than the cases (misclassification bias)

Front 87

What are the potential strengths to using a secondary study base in a case-control study?

Back 87

-Less expensive than a primary base (no random-digit dialing)
-Controls often have another illness (not healthy) and therefore more interested in participating and higher response rate
-Accuracy of information on determinant from controls may be similar to cases

Front 88

What are the potential limitations of using secondary study bases?

Back 88

-Difficult to know if controls represent the study base that produce the cases (come from the same source population)

-May be difficult to select control diseases unrelated to determinant of interest

Front 89

Odds Ratio

Back 89

(Index cases / index controls) / (reference cases / reference controls) = odds ratio

Front 90

Odds

Back 90

index cases / index controls = odds of illness among index (exposed)

reference cases / reference controls = odds of illness among referent (unexposed)

Front 91

What is the purpose of controls in a case-control study?

Back 91

-To provide information on the distribution of the determinant in the population experience that produced the cases. (proportion of people with and without determinant in study base)

-Provide information for denominators which is necessary for calculation of odds ratio

Front 92

What is the purpose of a placebo group in an RCT?

Back 92

The placebo group provides an estimate of the frequency of the outcome in the reference group.

Front 93

What are the two main criteria for valid control selection in a case-control study?

Back 93

-Controls come from the source population that produced the cases; if they develop illness, would be a case in the study

- Controls are selected INDEPENDENT OF EXPOSURE

Front 94

AF for case control study?

PAF?

Back 94

AF = (OR-1)/OR

PAF = Pe*(OR-1)/OR
Pe = # of exposed cases/total number of cases

Front 95

What are the potential strengths of case control studies?

Back 95

-Efficient for rare diseases or for diseases with a long induction period
-Efficient when exposure information is expensive
-Can evaluate multiple exposures for a single illness

Front 96

What are the potential limitations of case-control studies?

Back 96

-Inefficient for rare exposures (??)
-Can be susceptible to selection bias (controls not representative of study base)
-Can be susceptible to information bias (if determinant information inaccurately obtained)
-Can't calculate cumulative incidence and incidence rate