relative risk, incidence proportion ratio incidence proportion in exposed / incidence proportion in unexposed = (exposed new cases / total exposed) / (unexposed new cases / total unexposed) = (A / A+C) / (B / B+D)

Clinicometrics Flashcards by Jessica Hodson

crude mortality

total # of deaths in a population / entire population

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cause specific mortality

deaths from disease / entire population

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case fatality mortality

deaths from disease / # of cases of disease

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proportionate mortality for disease

deaths from disease / total # deaths in a population

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prevalence

total number of cases in a population at a given time

cases at time T / population at time T

does NOT determine risk or probability of becoming a case

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what type of study determines prevalence

cross sectional studies

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can CS studies calculate risk/probability

no because it ignores disease duration

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what factors influence prevalence

population dynamics
duration of disease

prevalence = incidence x disease duration

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incidence

risk/probability of disease occurring in a time period

used to compare disease occurrence in one group versus another (exposed vs not exposed OR treatment vs controls)

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what type of study determines incidence

clinical trials
cohort studies

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what is required in order to calculate incidence

defined time interval (study length)

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incidence proportion

new cases during study period / susceptible subjects

ranges from 0 to 1

assumes subjects do not have disease at the start of the time period and are not at risk of being removed from the population

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limitations of calculating incidence proportion

diseases with recurrence
dynamic populations - requires calculation of incidence rate

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incidence rate

new cases during study period / time that subjects are at risk of disease

denominator = sum of time at risk for all individuals in the population
- accounts for dynamic populations

measured in units of time (cases per X number of days)

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2X2 tables - A, B, C, D

A: diseased + exposed
B: diseased + not exposed
C: healthy + exposed
D: healthy + not exposed

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risk ratio (RR)

incidence proportion in exposed / incidence proportion in unexposed

relative risk, incidence proportion ratio

(# exposed new cases / total # exposed) / (# unexposed new cases / total # unexposed)
=
(A / A+C) / (B / B+D)

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how do you interpret risk ratio

RR probability of developing disease in exposed vs unexposed

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range of risk ratio

0 to infinity

RR > 1: exposure is a risk factor for disease
RR < 1: exposure is protective against disease

risk difference

incidence proportion in exposed - incidence proportion in unexposed

(A/ A+C) = (B/ B+D)

absolute measure of risk
determines the importance of the disease

interpretation of risk difference

for every (total # exposed), only X develop disease due to exposure

incidence rate ratio

incidence rate in exposed/ incidence rate in unexposed

(# exposed new cases / time exposed are at risk) / (# unexposed new cases / time unexposed are at risk)

interpretation of incidence rate ratio

patients will get disease IRR times faster if exposed

hazard rates

similar to incidence rate but estimated from a type of statistical model that estimates time to event of interest in different groups

cohort study uses

good for COMMON diseases to calculate RISK RATIO

start with population at risk (disease free) –> determine exposure

why can you calculate risk ratio of a cohort study

because samples are randomly selected (all subjects are disease free at enrollment)

limitations of cohort studies

rare diseases make it difficult because you would have cases <<< controls inefficient to follow all controls just to find an adequate number of cases - can compare the cases to a smaller sample of controls and statistically still get the same results

case control study uses

similar enrollment as cohort study except controls are sampled from the cohort - allows study of RARE diseases cases: enrolled from the cohort if they develop the disease over a set time period controls: enrolled from the cohort if they are at risk of getting the disease but do not have it yet

can you calculate risk ratio from a case control study

NO because the number of cases and controls were selected by the investigator (not randomly sampled like cohort) subjects are STARTING with disease vs no disease

case control study steps

1. start with a cohort 2. determine the marginal totals of cases and controls (sampled from cohort) 3. determine exposure status of cases and controls

what can be calculated from case control studies

- odds of exposure - case control (exposure) odds ratio

odds of exposure

probability of being exposed / probability of not being exposed = p / 1-p where p = probability

odds of exposure among cases

proportion of cases exposed / proportion of cases not exposed = (A / A+B) / (B / A+B) = A / B

odds of exposure among controls

proportion of controls exposed / proportion of controls not exposed = (C / C+D) / (D / C+D) = C /D

case control odds ratio

exposure odds ratio odds of exposure among cases / odds of exposure among controls = (A/B) / (C/D) = AD / BC

can risk ratio be calculated from a case control study

NO - but can be approximated from the case control odds ratio risk ratio = disease odds ratio = case control odds ratio ONLY when the disease is rare

can you calculate risk from a case control study

NO and you can NOT approximate it either can only approximate RISK RATIO (relative risk)

exposure odds ratio vs disease odds ratio

EOR: measured in case control studies DOR: measured in cohort studies

how are EOR and DOR related

EOR is always equal to DOR

how are DOR and risk ratio related

DOR = risk ratio (relative risk) when the disease is RARE

when is a disease considered rare

when risk < 5% in a period of time odds is bound from 0 to infinity risk is bound from 0 to 1 when p = 0.05 --> odds curve diverges from probability/risk curve --> odds ratio no longer equals risk ratio

how are controls selected for a case control study

3 rules 1. control should only be selected if someday, if it were to become a case, then it would be eligible to be included in the study as a case 2. controls should be selected for reasons unrelated to exposure 3. select incident (new) cases NOT prevalent (old) cases

how many controls should be enrolled in a case control study

want to have a minimum of 1 control per case more controls per case is better, but no need for >4 cases per control

what is the use of case control studies

hypothesis generating - examine a large number of exposures for one outcome/disease