collects data from every individual in the population
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Sample
subset of the population from which we collect data
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sample survey
a study that collects data from a sample to learn about the population from which the sample was selected
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Conveince Sample
Selecting people from the population that are easier to reach
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Bias
A study that is likely over or underestimate the true data
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voluntary response sample
Allows people to choose to be in the sample by responding to a general invitation
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random sampling
using a chance process to determine which members of a population are included in the sample
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simple random sample (SRS)
of size n consists of n individuals from the population chosen in such a way that every set of n individuals has an equal chance to be the sample actually selected
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sampling without replacement
an individual can only be selected once
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sampling with replacement
an individual from a population can be selected more than once
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Strata
Groups of individuals in a population that are similar in some way that might affect their responses.
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stratified random sampling
choosing an SRS from each strata combining the SRSs into one overall sample.
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cluster
group of individuals in the population that are located near each other
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Systematic random sampling
randomly choose one person and survey them and every kth person after them
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Cluster sampling
randomly select a cluster and survey everyone in the clusters
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Undercoverage
occurs when some groups in the population are left out of the process of choosing the sample
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Nonresponse
occurs when an individual chosen for the sample can't be contacted or refuses to participate
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response bias
systematic pattern of inaccurate answers to a survey question
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observational study
observes individuals and measures variables but does not influence the response
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response variable
measures the outcome of a study
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explanatory variable
a variable that we think explains or causes changes in the response variable
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confounding
When two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other.
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Experiment
A research method in which an investigator manipulates one or more factors to observe an outcome
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placebo
a treatment that has no active ingredient, but is otherwise like other treatments
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treatment
a specific condition applied to the individuals in an experiment
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experimental unit
the object to which a treatment is randomly assigned
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subject
experimental unit that is a person
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factor
a variable that is manipulated and may cause a change in the response variable (usually used when talking about multiple variables)
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levels
the specific values that the experimenter chooses for a factor (ex. yes or no, or 10th grade, 11th grade, or 12th grade)
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Control group
used to provide a baseline for comparing the effects of other treatments
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placebo effect
some subjects will respond to any treatment, even if inactive
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double-blind
neither the subject nor person administering experiment knows what treatment the subject is getting.
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single-blind
An experiment in which either the subjects or those who interact with them and measure the response variable, but not both, know which treatment a subject received.
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random assignment
assigning participants to experimental and control conditions by chance, thus minimizing preexisting differences between those assigned to the different groups
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control (similar to control group)
keeping other variables constant
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replication
giving each treatment to enough experimental units so that a difference in the effects of the treatments can be distinguished from chance variation.
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4 principles of experimental design
Comparison
Random Assignment
Control
Replication
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Completely randomized design
units are assigned to treatments completely at random
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Block
a group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments
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Randomized block design
random assignment is carried out within each block
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Matched pairs design
experimental design that uses blocks of size of two
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sampling variability
The fact that different random samples of the same size from the same population produce different estimates
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statistically significant
an observed effect so large that it would rarely occur by chance
Studied by 105 people
