Lecture 12 - Statistics And Probabilities Flashcards
the science of epidemiology rests on ______
statistics
ALL of public health relies on _______ to provide and interpret data
statistics
the term “statistics” refers to both…….
-the numbers that describe the health of the populations
-The science that helps to interpret those numbers
the science of statistics is a set of ____ and ____ used to ____ ____ in order to ____ _____
the science of statistics is a set of CONCEPTS and METHODS used to ANALYZE DATA in order to EXTRACT INFORMATION
______ results from epidemiological studies are common
contradictory
What are 2 sources of error?
-Biasing (unintentionally guiding the results in one direction)
-confounding variables (very difficult to control all variables)
Give an example that demonstrates that people sometimes demand certainty about their public health even when science cannot prove it
1997 - mammography screening - NCI said that each woman should make the decision individually with their dr to be screened or not based on medical and family history
-public response was heated
in the end, NCI recommended that women in their 40s be screened —pressure from politicians who were eager to get credit for supporting women’s health led to a misguided sense of SCIENTIFIC CERTAINTY
politics vs science
When there is uncertainty in epidemiology…. _____ may prevail. Why?
politics
politicians pressure scientists to change their opinion (ie: the breast cancer screening)
What is the issue with politicians convincing scientists to change their mind on things?
the public finds it frustrating to hear contradictory information when they’re genuinely concerned about protecting their health
Science always involves _____
uncertainty
How do scientists quantify uncertainty?
by measuring probabilities (requires statistics)
think of “probability” as….
the likelihood that an event of this type will occur based on what has happened in the past
______ said that the _____ is what usually happens. BUT statisticians know that the ______ happens more than most people think
ARISTOTLE said that the PROBABLE is what usually happens. BUT statisticians know that the IMPROBABLE happens more than most people think
What is the P value?
the likelihood that the difference between control and experiment is due to variability.
or the degree of probability or improbability of a certain result in an experiment
the p value expresses the probability that…..
the observed result could have occurred by chance alone
TYPICALLY, a p value of ____ or less is acceptable
0.05
what does a p value of 0.05 mean?
if an experiment were repeated 100 times, the same answer would result 95 of those times, while 5 times would yield a different answer
Give a reason that an acceptably low p value could lead to an erroneous conclusion
-bias
-confounding variables
bias and confounding variables are ____ errors
systemic (innate)
Give an example of a specific case in which the p value was statistically significant (0.001) but the results were erroneus
the study that found a link between pancreatic cancer and coffee drinking.
the conclusion is wrong NOT due to random chance, but because the cancer was caused by smoking rather than coffee drinking
What is the reason that large numbers of cancer treatments could be in clinical use that are not effective?
p value of 0.05 indicates that 5 out of every 100 ineffective treatments would appear to be effective, but may actually represent errors caused purely by chance
The fact that the probable is not always what happens leads to the law of _______
law of small probabilities
What does the Law of Small Probabilities state?
the most improbable things are bound to happen occasionally
Due to the law of small probabilities, a few people with apparently fatal diseases will inexplicably recover and may be convinced that their recovery was the result of……
something that they did, giving rise to a new quack therapy. Because their recovery was merely a random deviation, other patients will not get the same benefit
Another consequence of the law of small probabilities is the phenomenon of….
cancer clusters
Explain how cancer clusters are a consequence of the law of small probabilities
every now and then a community will discover that it is the site of some kind of cancer and everyone will be alarmed
-not the result of an environmental agent, but is the result of chance variation
If a cancer cluster is very large, however…..
it is likely NOT random variation
a large number of cases is said to confer _____ on a study
power
power increases the probability of…..
finding an effect (if there is one)
___ and ____ can be present in even the largest of studies
bias and confounding variables
Studies with low power are likely to produce……
false negative results
false positive results occur when…
the study finds an effect that is not real (when random variation appears to be a true effect)
In the mission of public health to prevent disease and disability, _________ and ______ plays an important role
secondary prevention (early detection) and treatment plays an important role
When the causes of a disease are not well known, little is known about _______
(ex: breast cancer)
primary prevention
When little is known about primary prevention of a disease, what is the next best public health measure?
to screen the population at risk so as to detect the disease early
Give an example of 2 populations involved in screening to detect disease early
identifying individuals with HIV and counseling them to prevent spreading to others
newborns are routinely screened for certain congenital diseases
Screening tests may be highly _______ resulting in….
highly sensitive, resulting few false negatives
Most lab tests should be highly accurate, but most are likely to yield ____ or ____ to some degree
false positives or false negatives
for most public health screening programs, _______ are desirable to avoid missing an individual with a serious disease who could be helped by some intervention
sensitive tests
Sensitive, inexpensive tests are chosen to encourage testing of as many at-risk individuals as possible are often not very______
specific
When a positive result is found using a non specific test, what happens next?
more specific tests are conducted to confirm the initial finding
if a SENSITIVE mammogram finds a suspicious spot in a woman’s breast, the test is usually followed by….
a biopsy to determine whether or not the spot is cancerous
true positives in the cases of rare diseases are _____.
rare
in the case of a rare disease, the rate of false positives may be _____ than the number of true positives when sensitivity tests are used
HIGHER
give a specific case in which false positives were a big issue
when louisiana mandated premarital screening for HIV
Describe what a sensitivity test is
used INITIALLY to screen people for disease. Results in many false positives.
What is the goal of sensitivity tests?
to make sure that everyone that actually has the disease is identified. (ensure that there are NO false negatives)
What are specific tests?
they are the follow up test after testing positive using a sensitivity test. more expensive, complicated, and can be invasive. If you were a FALSE POSITIVE, this test will most likely show that you dont have the disease
what is a rate?
percentage of occurrence in a population
epidemiology makes extensive use of rates to….
study disease distribution and determinants
Raw data needs to be converted to ____ to be valuable
rates
birth rate (like the death rate) is defined as….
while the fertility rate is defined as….
the number of live births per 1000 people
fertility rate = number of live births per 1000 women AGES 15-44
Both rates start with the same raw number (number of live births) but use a DIFFERENT POPULATION FOR REFERENCE
What are 2 rates commonly used as indicators of a community’s health?
infant mortality rate
maternal mortality rate
what is the infant mortality rate?
the number of infants that dies before their first birthday in a year, divided by the number of live births in that year
what is the maternal mortality rate?
the number of deaths among women associated with pregnancy divided by the number of live births in that year
For some purposes, the numbers can be made more useful by converting crude rates to adjusted rates. give an example
crude mortality rate in florida is much higher than the crude mortality rate in alaska —-no cause for alarm: the average age of a floridian is higher than the alaskan
after adjusting the mortality rate for age differences, the mortality rates are similar
rates can be adjusted based on ___, ____, ____, and -___
age, gender, race, and ethnicity
Rates can be calculated on a _____ specific basis
group
who has a higher mortality rate - males or females??
males
What is YPLL
years of potential life lost
YPLL gives greater weight to the deaths of…..
young people
What are leading causes of YPLL?
Accidental injuries, infectious disease, and suicide because they are likely to kill young people
The formal process of risk assessment…..
identifies events and exposures that may be harmful to humans and estimates the probabilities of occurrence as well as the extent of the harm they may cause
assessing risk is often done on the basis of……
historical data
assessing risk is often done on the basis of historical data. give an example
can predict that the # of motor vehicle crashes next year will be similar to the number this year
To estimate the probability of a mishap in new technology, various possible chains of events are considered, and the…..
risk for something going wrong is estimated in each step
risks of the individual steps are then added or multiplied to obtain a risk for the whole. give a specific example of when this was done
when nuclear powerplants were first introduced - helped engineers identify what kind of safety devices should be incorporated to reduce the probability of failure
According to an analysis published in 2000, experts said that the most risky activities and technologies were:
-motor vehicles
-smoking
-alcoholic beverages
-handguns
-surgery
however, when the general public was asked about their perception of risk, what was at the top of their list?
nucelar power
people are more likely to accept a risk from an activity when….
it is perceived to be beneficial
in terms of the public, what are the 3 ways in which they assess risk?
-familiar risks are more acceptable than unfamiliar risks
-risks that people perceive they have control over are more acceptable than those that are uncontrollable
-a risk with potentially catastrophic consequences is unacceptable, even if it is highly unlikely to occur
Risk perception researchers classify risks on 2 scales:
dreadness
knowability
in terms of automobiles,
driving is one of the riskiest activities, yet it does not arouse anxiety because it is neither dreaded nor unknown. people also perceive that they have control when they’re driving - benefit is obvious
Why is a nuclear reactor perceived as a great risk by the public while driving is not?
highly dreaded, lack of control, benefit is unclear
When did the public’s perception about nuclear power gain credibility?
after the 1979 accident at the 3 mile island nuclear reactor in pennsylvania
What is the difficulty with a cost-benefit analysis when it comes to public health?
determining what monetary value to place on a life saved
What is a cost benefit analysis?
What is a cost effectiveness analysis?
cost benefit analysis = weighs the estimated cost of implementing a policy against the estimated benefit, usually in monetary terms
cost effectiveness analysis = compares the efficiency of different methods of attaining the same objective
Should cost-benefit analysis and cost-effectiveness analysis serve as the sole determinant of a health care decision?
no - but it can improve decision making
