Ch 6 - Samples and observational studies Flashcards
Observational study
Record data on individuals without attempting to influence the responses.
describing existing groups and situations
Ex. Do female crickets choose their mates on the basis of their health? Observe health of male crickets that mated.
Experimental study
Deliberately impose a treatment on individuals, and record their responses. Influential factors can be controlled. Individuals do not chose which treatment they get.
more control
Ex. Infect some males crickets with intestinal parasites and keep other males healthy. Set traps to see whether females tend to choose healthy rather than sick males.
Two variables are confounded when
their effects on a response variable cannot be distinguished.
Observational studies often fail to yield clear causal conclusions, because
the explanatory variable is confounded with lurking variables.
1) describes the results of observational studies. Access to medical care, lifestyle, socioeconomic status, are possible confounding variables.
2) describes experiments, with a random assignment of subjects to treatment.
Population versus sample
How do you select the individuals/units in a sample?
Bias
if systematically favor one outcome
A Simple Random Sample (SRS) is made of _____, each ______
All possible samples of size n have ____
randomly selected individuals.
Each individual in the population has the same probability of being in the sample.
the same chance of being drawn.
How to choose an SRS?
Choosing a simple random sample with Table A
A stratified random sample is
the combination of two or more SRSs taken from subgroups of a given population.
The subgroups are chosen to contain all the individuals with a certain characteristic.
Multistage samples are
drawn in multiple stages, by taking a random sample within a random sample
A sample survey is
an observational study that relies on a random sample drawn from the entire population.
survey challenges
How bad is nonresponse
Method: First survey is conducted via phone, whereas the second survey is conducted online. Online surveys tend to have younger participants.
Wording: First survey does not define the theory, while the second does. The first survey also uses somewhat confusing wording “personally believe” (as opposed to what kind of belief?).
Case-control studies
start with 2 random samples of individuals with different outcomes, and look for exposure factors in the subjects’ past (“retrospective”).
Cohort studies
enlist individuals of common demographic, and keep track of them over a long period of time (“prospective”). Individuals who later develop a condition are compared with those who don’t
Case-control studies vs Cohort studies
