a procedure by which EVERY MEMBER of a population is selected for study
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representative sample
a sample w/ the essential characteristics of the population studied without any bias
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sampling frame
list of all the members of population from which we can draw a sample
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probability sample
a sample in which every individual in the population has a known probability of being sampled
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simple random sample
a sample in which ALL samples of size x have an equal probability of being drawn
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systematic sample
a sample in which the starting point is chosen randomly, but afterwards the sample is drawn from some predetermined system/pattern (e.g. every 12th person)
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stratified sample
sampling frame is divided into homogeneous groups, then random sample is drawn from each homogeneous group
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cluster sample
sampling frame is divided into groups which are all the same/intended to be representative of the population; then, clusters are sampled randomly
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self-selected/voluntary response sample
a sample in which people choose whether or not to participate in the survey/respond (TENDS TO BE BIASED, VOLUNTARY RESPONSE BIAS)
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convenience sample
sample drawn however is most convenient (TENDS TO BE BIASED)
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quota sampling
individuals are selected intentionally to try to reflect different quotas of groups in the population, e.g. selecting x amount of teens to include because teens make up y proportion of the population (TENDS TO BE BIASED)
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bias
the tendency of a sample statistic to systematically over- or under-estimate a population parameter (e.g. our sampling method will regularly & systematically produce samples that misrepresent our data in the same way)
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undercoverage
one part of the population being sampled is excluded/underrepresented
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voluntary response bias/nonresponse bias
certain individuals (those who feel certain ways about the subject matter of the survey) will choose not to respond/to respond; the groups of individuals who respond/don’t respond are the same w/ respect to the variable of interest & thus we’ve under/overrepresented a group in our population
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wording bias
bias which occurs when the wording of the question(s) being asked influences the responses in a SYSTEMATIC way
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what are some situations in which we might see response bias?
the respondent might not want to give a truthful answer to the question (e.g. income questions); the respondent might not UNDERSTAND the question (ambiguous wording); the respondent might want to please the interviewer; the order in which options were listed might influence the answers of the respondent; etc.
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a difference between treatment results/treatment & no treatment results is statistically significant if…
this difference is too large to attribute to chance/natural random variation
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experiment
a study in which researchers ASSIGN individuals to treatments/impose treatments on individuals
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observational study
a study in which researchers observe individuals and record data but DON’T impose treatments
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retrospective studies study what is in the ____; prospective studies will study events in the…
past; future
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confounding variable
a variable that has an effect on the outcomes of the study but whose effects can’t be separated from those of the treatment variable
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statistical control
a situation in which all variables other than the one of interest are HELD CONSTANT across all treatment groups
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what are the 3 basic principles all experiments must incorporate?
randomization, replication, and control
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randomization is done to…
equalize groups so that the effects of extraneous variables are equally distributed among groups
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replication is done to…
minimize effects of random chance on our results (e.g. ensure differences between results for our groups aren’t due to random extremity)
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control is enforced to…
minimize confounding; ensure groups only differ with respect to variable of interest
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completely randomized design
subjects randomly assigned to treatment groups, treated; effects of treatments compared
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placebo
an inactive treatment used when effects of treatment are compared to effects of no treatment (placebo only used for HUMAN subjects)
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blinding
preventing a party in the experiment from knowing which treatment they received (single blind: either subject or evaluator doesn’t know; double blind: both subject and evaluator don’t know)
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when describing a method of randomization, be sure to…
specify method of randomization, specify whether sampling w/ or w/out replacement, and (if using random digits table) specify what represents 1 \[e.g. 01 is 1\]
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randomized block design
completely randomized experiment is performed WITHIN each block, results compared WITHIN blocks
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matched-pairs design
two subjects similar to each other are paired, becoming a block, or one person becomes a pair & their before/after results are compared
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