Identifying the cause of a disease

Question 1

Cohort studies

Answer 1

estimating risks of acquiring a disease and used for determining whether there is an association between a factor or characteristic and disease development

Question 2

Compare cohort and case-control studies

Answer 2

Cohort

• exposed and unexposed persons are compared
• compare incidence of disease in exposed and unexposed individuals
• disadvantages: require large populations
• prospective cohort is expensive to carry out because of follow up of large populations
• potential bias is greater
• become impractical when disease under study is rare

Case control

• persons with or without disease compared
• compare proportions who have the exposure of interest in people with or without disease
• advantages: relatively inexpensive
• require small number of subjects for study
• desirable when disease occurrence is rare
• potential for recall bias

Question 3

How to design a cohort study

Answer 3

estimating incidence of disease (rate of death from disease)
1- investigator selects two or more groups of individuals
one group has been exposed to risk factor or disease and the other has not
Both groups are then followed and the development of the disease in the two groups is compared
all individuals in the group are observed until they either die or become a case or end of the study
If the incidence in the exposed group is higher then in the non-exposed there is a positive association between exposure and disease

Question 4

Comparing cohort studies and randomized trials

Answer 4

Both compare exposed and non- exposed groups. Difference is presence or absence of randomization

Question 5

Two ways to generate these two groups

Answer 5

• select groups on basis of whether or not they were exposed, so study begins with exposed and non-exposed
• select a defined population before any of its members become exposed or before their exposures are identified

Question 6

Types of cohort studies

Answer 6

Prospective cohort

Retrospective cohort

Question 7

Prospective cohort

Answer 7

investigator identifies study population at the beginning is the study and monito the subjects through time until disease develops or does not develop.
The researcher starts study at the time when it is realized that the cohort has been exposed to something or monitors when exposure occurs in a group of people
The investigator then determines current status of the individuals and selects only those that have not yet developed disease and follows them through time
Drawback because follow up , the study can take many years to complete. People lost to follow up owing to improved methods of detecting exposure there may be changes in classification of individuals

Question 8

Retrospective cohort

Answer 8

Investigator uses historical data of all the exposed and non-exposed subjects and then assesses their case or non-case status
Study is started long after the exposure time and after the disease has established itself in the cohort study being studied
Drawbacks are selection bias and misclassification of individuals as historical data is being used

Question 9

Potential biases in cohort studies

Answer 9

bias in assessment of outcome
information bias
bias in non-response and losses to follow up
analytic bias

Question 10

Factors to be considered when dealing with large cohort

Answer 10

At what point should the individuals in the cohort be identified?
Should the cohort be drawn from one place or from a few places or should a national sample be drawn?
For how long should the cohort be followed?
What hypothesis or how many hypotheses should be tested on the cohort

Question 11

When is a cohort study warranted

Answer 11

When there is enough evidence to suggest an association of a disease with a certain exposure.

Question 12

Study designs that can be used to study etiology

Answer 12

case-control studies
case crossover design
cross-sectional studies

Question 13

Case control study

Answer 13

Compare to determine whether a certain exposure is related to the risk of developing some condition. People with disease are compared with those who don’t

Question 14

Case control study steps

Answer 14

identify a group of individuals with disease and a group without
determine what proportion of cases were exposed to a particular factor and what proportion were not
Determine what proportion of the controls were exposed to the same factor and what proportion were not

Question 15

Four groups in case control study

Answer 15

cases that were exposed
cases that were not exposed
controls that were exposed
controls that were not exposed

Question 16

Why data from case control study cannot be used to estimate prevalence of disease

Answer 16

investigator determines no. of controls per case therefore the proportion of the study population is determined by the investigator and does not reflect the true prevalence of the disease in the population

Question 17

Selection of cases and controls

Answer 17

cases can be collected from hospitals, clinics or private physician patients.
When selecting cases ensure risk factors are generalizable to all patients with the disease and the criteria must be clearly stated and specifies in writing
Investigator can use incidence or prevalent cases

Question 18

Sources of controls

Answer 18

non-hospitalized people
hospitalized patients (with diseases other than the one being studied
- school rosters
insurance company lists
best friend, spouse or sibling

Question 19

Problems in control selection

Answer 19

might have very high or very low level of exposure that might not be the representative of the level in the population of study

Question 20

Problems of memory

Answer 20

Exposure data collected, the quality will depend on the ability of the cases and controls to recall any exposures
During case-control studies data related to exposure is collected from subjects in interviews but this data may not be accurate because:
most humans are limited in their ability to remember info
subjects may not have the info being requested
this could lead to misclassification if subjects don’t know their exposure accurately others will be classified as unexposed when they were in fact exposed and vice versa

Question 21

Matching cases and controls

Answer 21

must have similar characteristics such as age, sex and socioeconomic status
Matching is a method used to ensure that the cases and controls are similar in certain characteristics or exposures that might influence a factor that is being studies

Question 22

Two types of matching

Answer 22

Group matching: cases selected first then controls so that the % of controls with a certain characteristic is identical to the % of cases with the same characteristic
Individual matching : a control is selected with similar characteristics for each case

Question 23

Problems with matching

Answer 23

if too many variables are matched it may be impossible to find an appropriate control
Unplanned matching that may bias the control group can also occur

Question 24

Cohort based control studies

Answer 24

Nested case control: as each case of the disease develops a control is selected from the individuals who are at risk for the disease at the time therefore cases and controls are matched on a calendar time and length of follow up
Case cohort studies: controls are not individually matched to each case but are randomly chosen from the defined cohort with whom the study began

Question 25

Case crossover design

Answer 25

similar to planned crossover
eliminates cost of identifying and interviewing separate control population
primarily used to study etiology of acute outcomes
It is used mainly to examine the effects of transient exposures( such as air pollution) on the risk of acute illness (such as asthma). Each case acts as its own control just at a diff time under diff conditions. Therefore all subjects are cases and the chances of experiencing an acute-onset disease is assessed under two diff conditions and exposures. It is assumed that if an exposure or event is linked to a disease onset then the exposure or event should happen more often just before disease onset than at times when there is no acute disease
This type of study examine whether the risk factor was present or absent immediately before the outcome or disease was experienced and whether the same risk factor was [present when the outcome was not experienced. This will clarify whether there is a link between risk factor and outcome

Question 26

Cross sectional studies aka prevalence study

Answer 26

use when u don’t know duration at time of study
the disease and exposure status are measured simultaneously for each subject providing a snapshot of the frequency and characteristics of a disease in a study population at a particular point in time
Provides weak evidence of causal association between exposure and outcome because the exposure may not have occurred before the onset of the disease. Cohort and case control studies need to be conducted to establish the causal association between exposure and disease or outcome