Bias & Sampling
In this sampling method, the population is divided into homogenous subgroups before randomly selecting members from each group proportionally.
What is Stratified Random Sampling?
This is the term for the baseline group in an experiment that does not receive the active treatment.
What is a control group?
This is the statistical term for the long-run average outcome of a random variable over many independent repetitions.
What is the expected value (or mean)?
Two events, A and B, are described by this term if P(A and B) = P(A) * P(B).
What are Independent Events?
A news station asking its viewers to call in and vote on a political issue suffers from this specific type of bias.
What is a voluntary response bias?
An experiment uses this specific design when neither the subjects nor the evaluators know who received the active treatment versus the placebo.
What is a double blind experiment?
If X has a mean of 4 and standard deviation of 2, these are the respective mean and standard deviation of the transformed variable Y = 3X - 5.
What is 7 and 6?
Given P(A) = 0.4, P(B) = 0.3, and P(A and B) = 0.1, this is the value of P(A or B).
What is 0.6?
This is the exact number of upperclassmen that should be selected when a stratified random sample of 200 is taken from a school of 1,000 students, of which 600 are upperclassmen.
What is 120?
Drawing names from a hat to assign 60 patients to three different drug dosages relies on this fundamental principle of experimental design to equalize lurking variables.
What is random assignment?
If independent random variables X and Y have variances of 9 and 16 respectively, this is the standard deviation of X + Y.
What is 5?
If 60% of a class passed math, 50% passed English, and 30% passed both, this is the probability that a student passed English given that they passed math.
What is 0.5?
Studying teen sleep habits by surveying only students who arrive at school before 7:30 AM is an example of convenience sampling and introduces this type of bias by completely missing late arrivers.
What is undercoverage bias?
Unlike an observational study, a well-designed controlled experiment allows researchers to establish this type of relationship between variables.
What is causation?
For a random variable X where P(X=1) = 0.2, P(X=2) = 0.5, and P(X=3) = 0.3, this is the expected value, E(X).
What is 2.1?
Drawing 2 marbles without replacement from a bag of 5 red and 3 blue marbles yields this exact probability that both marbles drawn are red.
What is 5/14?
This type of bias occurs when individuals chosen for a sample cannot be contacted or refuse to participate, and their opinions differ from those who do respond.
What is nonresponsive bias?
In an experiment testing three fertilizers on tomatoes, keeping the amount of water and sunlight exactly the same for all plants is an application of this core experimental design principle.
What is control?
Because standard deviations do not add directly for independent random variables, you must instead add these together before taking the square root.
What are the variances?
A disease affects 1% of the population. A test has a 95% true positive rate and a 90% true negative rate. Using Bayes' theorem, this is the approximate probability a person testing positive actually has the disease.
What is 8.76%?