Week 3: Measures of disease frequency, effect and potential impact Flashcards
The three general classes of mathematic parameters are what? What are they used for?
Ratios, Proportions, Rates.
Often used to relate the number of cases of a disease (numerator) or health outcome to the size of the source population (denominator) in which they occurred
How would you calculate the number of stillbirths per thousand live births?
Number of stillbirths / # livebirths x 1000
The number of fetal deaths out of the total number of births and stillbirths per hundred live births is a _____.
Proportion.
What is risk? What are its two assumptions?
Probability that an event will occur within a stated period of time.
Does not have disease at start of follow-up. Does not die from other cause during follow-up (no competing risks).
Risk can be estimated directly and indirectly from what?
Cumulative Incidence (directly); sometimes called “incidence risk”.
Incidence density (indirectly, e.g., via life tables).
How do you calculate the rate of new cases of Parkinson’s disease which develops per 1,000 person-years of follow-up?
Number of new Parkinson’s cases / Total time disease-free subjects observed x 1000
What is incidence? What is a pro and a con of it.
Measures new cases of a disease that develop over a period of time.
Very helpful for etiologic/causal inference (follow-up over time); difficult to estimate.
Define prevalence. Provide two benefits and the typical associated study design.
Measures existing cases of a disease at a particular point in time or over a period of time.
Very helpful for quantifying disease burden (e.g. public health). Relatively easy to estimate. Often implies a cross-sectional design.
What is cumulative incidence? How is it calculated?
Measures the frequency of addition of new cases of disease and is always calculated for a given period of time (e.g. annual incidence) Must always state the time period (since time is not automatically captured). Also considered a measure of average risk.
Number of new cases during follow-up / # of disease-free subjects at start of follow-up
What are the problems with cumulative incidence over long periods of observation?
What to do about losses to follow-up? Risks may not be constant over long follow-up, and thus, the follow-up time needs to be partitioned.
What is incidence density (incidence rate)? How is it calculated?
Measures the rapidity with which new cases are occurring in a population Most sophisticated form of measuring incidence - accounts for losses, competing risks, dynamic turn-over, differential follow-up time, changes in exposures over time.
Number of new cases during follow-up / total time that disease–free individuals in the cohort are observed over the study period (total person-time experience of the cohort).
What is the measure? Rates of obstetric acute renal failure rose from 1.66 to 2.68 per 10000 deliveries between 2003-04 and 2009-10…
A cumulative incidence.
What is the measure? Maternal mortality increased from 7.55 deaths per 100 000 live births in 1993 to 21.5 deaths per 100 000 live births in 2014.
A ratio: the numerator is independent of the denominator, e.g. the maternal death may have resulted in a stillbirth.
What is the measure? First breast cancer diagnosis among those receiving a prescription of either standard dose or high dose folic acid for neural tube defect prevention, over the person-time from start of the first prescription.
Incidence density (incidence rate): new cases of breast cancer / person-years of follow-up time.
How do you calculate risk, risk differences, and risk ratio of disease given exposure?
Risk of disease in exposed: a / (a+b)
Risk of disease in non-exposed: c / (c+d)
Risk difference of disease given exposure: (a/(a+b)) – (c/(c+d))
Risk ratio of disease given exposure: (a/(a+b)) / (c/(c+d))
