FARR Epidemiology Flashcards
Bias introduced into a study when a clinician is aware of the patient’s treatment type.
Observational bias.
Bias introduced when screening detects a disease earlier and thus lengthens the time from diagnosis to death.
Lead-time bias.
If you want to know if geographical location affects infant mortality rate but most variation in infant mortality is predicted by socioeconomic status, then socioeconomic status is a _____.
Confounding variable.
The number of true positives divided by the number of patients with the disease is _____.
Sensitivity.
Sensitive tests have few false negatives and are used to rule _____ a disease.
Out.
PPD reactivity is used as a screening test because most people with TB (except those who are anergic) will have a
Highly sensitive for TB.
Cohort study—incidence or prevalence?
Incidence and prevalence.
Case-control study—incidence or prevalence?
Neither.
Describe a test that consistently gives identical results, but the results are wrong.
High reliability, low validity.
Difference between a cohort and a case-control study.
Cohort studies can be used to calculate relative risk (RR), incidence, and/or odds ratio (OR). Case-control studies can be used to calculate an OR.
Attributable risk?
The incidence rate (IR) of a disease in exposed – the IR of a disease in unexposed.
Relative risk?
The IR of a disease in a population exposed to a particular factor ÷ the IR of those not exposed.
Odds ratio?
The likelihood of a disease among individuals exposed to a risk factor compared to those who have not been exposed.
Number needed to treat?
1 ÷ (rate in untreated group – rate in treated group).
In which patients do you initiate colorectal cancer screening early?
Patients with IBD; those with familial adenomatous polyposis (FAP)/hereditary nonpolyposis colorectal cancer (HNPCC); and those who have first-degree relatives with adenomatous polyps (< 60 years of age) or colorectal cancer.
The most common cancer in men and the most common cause of death from cancer in men.
Prostate cancer is the most common cancer in men, but lung cancer causes more deaths.
The percentage of cases within one SD of the mean? Two SDs? Three SDs?
68%, 95.4%, 99.7%.
Birth rate?
Number of live births per 1000 population in one year.
Fertility rate?
Number of live births per 1000 females (15–44 years of age) in one year.
Mortality rate?
Number of deaths per 1000 population in one year.
Neonatal mortality rate?
Number of deaths from birth to 28 days per 1000 live births in one year.
Postnatal mortality rate?
Number of deaths from 28 days to one year per 1000 live births in one year.
Infant mortality rate?
Number of deaths from birth to one year of age per 1000 live births (neonatal + postnatal mortality) in one year.
Fetal mortality rate?
Number of deaths from 20 weeks’ gestation to birth per 1000 total births in one year.
Perinatal mortality rate?
Number of deaths from 20 weeks’ gestation to one month of life per 1000 total births in one year.
Maternal mortality rate?
Number of deaths during pregnancy to 90 days postpartum per 100,000 live births in one year.
False-neg ratio =
1 – sensitivity
False-pos ratio =
1 – specificity
Sensitivity =
a/(a+c)
Specificity =
d/(b+d)
PPV =
a/(a + b)
NPV =
d/(c + d)
High sensitivity is particularly desirable when
there is a significant
penalty for missing a disease. It is also desirable early in a diagnostic workup, when it is necessary to reduce a broad differential. Example: An initial ELISA test for HIV infection.
High specificity is useful for confirming
a likely diagnosis or for situa- tions in which false-
The positive predictive value (PPV) is
the probability that a patient with a
The negative predictive value (NPV) is
he probability that a patient with a
Advantages of cohort studies are as follows:
They follow the same logic as the clinical question (if people are ex-
posed, will they get the disease?).
I They are the only way to directly determine incidence.
I They can be used to assess the relationship of a given exposure to many
diseases.
I In prospective studies, exposure is elicited without bias from a known
outcome.
The disadvantages of cohort studies include the following:
They can be time consuming and expensive.
I Studies assess only the relationship of the disease to the few exposure
factors recorded at the start of the study.
I They require many subjects and are thus inefficient and cannot be
used to study rare diseases.
The validity of a case-control study depends on
appropriate selection of cases and controls, the manner in which exposure is measured, and the manner in which extraneous variables (confounders) are dealt with.
Advantages of such studies are as follows:
I Studies use small groups, thereby reducing expense.
I They can be used to study rare diseases and can easily examine mul-
tiple risk factors.
Disadvantages include the following:
Studies cannot calculate disease prevalence, incidence, or relative risk, because the numbers of subjects with and without a disease are deter- mined artificially by the investigator rather than by nature (an odds ra- tio can be used to estimate relative risk).
I Retrospective data may be inaccurate owing to recall or survivorship biases.
Absolute risk:
Defined as the incidence of disease.
Attributable risk (or risk difference):
The additional incidence of disease
that is due to a risky exposure, on top of the background incidence from other causes.
Attributable risk = Incidence of disease in exposed – Incidence in unexposed
Relative risk (or risk ratio):
Expresses how much more likely an exposed person is to get disease in comparison to an unexposed person. This indi- cates the strength of the association between exposure and disease, mak- ing it useful when one is considering disease etiology.
Relative risk = Incidence in exposed Incidence / in unexposed
Odds ratio:
An estimate of relative risk that is used in case-control studies. The odds ratio tells how much more likely it is that a person with a disease has been exposed to a risk factor than someone without the disease. Thelower the disease incidence, the more closely it approximates relative risk (see Figure 2.4-2).
A randomized controlled trial is
an experimental, prospective study in which subjects are randomly assigned to a treatment or control group.
Advantages of randomized controlled trials are as follows:
I They involve minimal bias.
I They have the potential to demonstrate causal relationships.
Disadvantages include the following:
They are costly and time intensive.
I Informed consent may be difficult to obtain.
I Some interventions (e.g., surgery) are not amenable to masking. I Ethical standards cannot allow all variables to be controlled.
Bias
Defined as
any process that causes results to systematically differ from the truth. Good studies and data analyses seek to minimize potential bias.
Types of bias include the following:
Selection bias: Measurement bias: Confounding bias: Recall bias: Lead-time bias: Length bias:
Type I (α) error:
Defined as the probability of saying that there is a difference in treat-
ment effects between groups when in fact there is not (i.e., a false-
Type II (β) error:
Defined as the probability of saying that there is no difference in treat- ment effects (i.e., a false-
If one is using a 95% CI, there is a
95% chance that the interval con-
tains the true effect size, which is likely to be closest to the point esti-
mate near the center of the interval.
Live attenuated
Measles, mumps, rubella, polio (Sabin), yellow fever, influenza (nasal spray).
Inactivated (killed)
Cholera, influenza, HAV, polio (Salk), rabies, influenza (injection).
Toxoid
Diphtheria, tetanus.
Subunit
HBV, pertussis, Streptococcus pneumoniae, HPV, meningococcus.
Conjugate
Hib, S. pneumoniae.
Prevention may be accomplished by
a combination of immunization, chemoprevention, behavioral counseling, and screening.
A good screen- ing test has the following characteristics:
It has high sensitivity and specificity.
I It has a high PPV.
I It is inexpensive, easy to administer, and safe.
I Treatment after screening is more effective than subsequent treatment
without screening.
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