OBSERVATIONAL STUDIESINTERVENTIONAL STUDIES

TUTORIAL

OBSERVATIONAL STUDIES Try to determine causes and risk factors

for disease by drawing connections between behaviors or exposures and diseases through observation alone.

Types of observational studies:1. Case report – observation made in a single

patient.2. Case series – observation that ties together

a group of patients.3. Cross-sectional study – observation of the

prevalence of a disease within a defined population.

4. Case control study – retrospective study in which causality is determined.

5. Cohort study – prospective study in which risk factors are followed to determine causality.

CASE CONTROL STUDY The prototypical retrospective

correlational studies. Compares a group of people with a

disease (cases) and a group of people without the disease (controls).

Then determines the degree to which each group was exposed to a particular risk factor.

The output of a case control study is the odds ratio.

Calculated as the probability of the risk factor among the diseased population divided by that in the healthy population.

OR = __ (cases exposed/cases unexposed)___

(controls exposed/controls unexposed)

COHORT STUDY The prototypical prospective

correlational studies. Identifies a group of people with a

particular potential risk factor (the cohort) and a group of people without the risk factor.

Then follows them into the future to see whether those with the risk factor contract the disease at a higher rate.

The output of a cohort study is the relative risk.

Relative risk is calculated as the incidence of the disease among the population exposed to the risk factor divided by the incidence of the disease in the unexposed.

RR = _(incidence in exposed) _ (incidence in unexposed)

INTERVENTIONAL STUDIES Attempts to establish a causal

relationship by manipulating one group with an intervention (treatment group) and observing what happens in comparison to another group that has not had the intervention or has had a placebo (control group).

a) Preventionb) Therapeutic

Phases of a clinical trial: Phase 1 – safety of the intervention is

tested on healthy individuals. Phase 2 – intervention is tested on

patients to determine the dosage and administration protocol .

Phase 3 – intervention is tested on a large group of patients, efficacy and side-effects are determined.

Phase 4 – after the intervention is made available, data is collected to evaluate safety and efficacy.

Blinding: Single blinded study – subjects are

unaware of whether they are in the experimental or control group .

Double blinded study – the subject and the observer are unaware of the subject’s group allocation.

Triple blinded study – the subject, observer and data analyst are unaware of the subject’s group allocation.

A new drug with in vitro activity against HIV is tested on a population of HIV-infected patients. Half of the patients are chosen by lottery to receive the drug. The drug which is tasteless, is administered in a cup of orange juice ; the other patients receive pure orange juice. Neither the nurses, doctors, nor patients know which patients receive the drug. At the end of the study period, the number of CD4 T-cells is determined for all of the subjects.

This is an example of :a) Case control studyb) Cohort studyc) Double-blinded randomized clinical

triald) Case report

A study to determine the relationship between elevated C-reactive protein levels and the development of acute coronary syndrome was performed. Data obtained after a 5 year follow-up revealed the following:

↑CRP Normal CRP

Total

Disease 10 20 30

No Disease 10 60 70

Total 20 80 100

According to the data, the RR of getting ACS in people with elevated CRP is:

a) 0.5b) 1.0c) 1.5d) 2e) 5Incidence of disease in people with ↑ CRP =

10/30 = 1/3Incidence of disease in people with normal

CRP = 20/30 = 2/3RR = 0.33/0.66 = 0.5

A group of investigators studied the relationship between the 11q22 translocation and DiGeorge’s Syndrome. A study population was randomly selected from newborn infants. Blood samples were obtained for genotyping and chest x-rays were performed to evaluate presence of thymus tissue. The group concluded that the 11q22 genotype is associated with DiGeorge’s syndrome. Which of the following correctly identifies the study design used by the investigators?

a) Case control studyb) Cohort studyc) Cross-sectional studyd) Case series

100 newborn children were studied for the risks of lead paint. 50 lived in homes with lead paint; of these 25 developed developmental delay within 5 years. 50 lived in lead-free homes; of these 5 developed developmental delay.

What type of study is this?a) Cohort studyb) Cross-sectional studyc) Case seriesd) Randomized clinical trial e) Case control study

What is the relative risk of developmental delay among children living in homes with lead paint?

Incidence of developmental delay among exposed = 25/50 = 0.5

Incidence of developmental delay among non-exposed = 5/50 = 0.1

RR = 0.5/ 0.1 = 5• Children living in homes with lead paint

are 5 times more likely to have developmental delay than children living in lead free homes.

A study was conducted to assess the association between smokeless tobacco and the development of lip cancer. Patients with lip cancer were asked about use of smokeless tobacco, and people without lip cancer that were randomly selected from the same population as the patients were asked about their use of smokeless tobacco. The study showed that use of smokeless tobacco has a significant association with lip cancer.

Which of the following measures of association are the investigators of the study most likely to report?

a) Relative riskb) Incidence ratec) Odds ratiod) Morbidity ratee) Positive predictive value

100 people with lung cancer were compared to 100 people without lung cancer. By history it was found that 90 individuals with lung cancer had smoked sometime in the past, while 25 of the healthy individuals had smoked.

What type of study is this?a) Cohort studyb) Cross-sectional studyc) Case seriesd) Interventional studye) Case control study

What is the odds ratio?Odds of smoking among those with lung

cancer = 90/10 = 9Odds of smoking among those without

lung cancer = 25/75 = 0.33OR = 9/0.33 = 27• Those with lung cancer had (past tense)

27 times greater odds of smoking (in the past) than those who don’t have cancer.

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