Reading Quiz 5 Flashcards
observational study
observes individuals and measures variables of interest but does not attempt to influence the response
experiment
deliberately imposes some treatment on individuals in order to observe their responses
can help eliminate (or at least try to minimize the effects of) lurking variables
population
the entire group of individuals you want information about
census
using every member of a population in a study (or at least attempting to contact every member)
sample
a subgroup of a population that you actually examine to gather information
sampling
involves studying a part in order to gain information about the whole
studying a population by taking a subset of it in order to generalize to the whole population
sampling error/variability
the natural variation one would expect to see in sample statistics from sample to sample
voluntary response sample
consists of people who choose to be part of a sample by responding to a general appeal
biased
convenience sample
involves choosing the most convenient individuals from the population for your sample
biased
bias
occurs when the sampling method systematically favors certain outcomes
the systematic error introduced when the sample is very different from the population
simple random sample (SRS)
unbiased
a sample of size n is selected in such a way that every individual in the population has an equal chance of being selected and every subset of n individuals has an equal chance of being selected for the sample
if there is a restriction on who can be chosen it is not an SRS
stratified random sample
includes the following steps:
1. divide the population into groups of similar individuals called strata
2. choose a separate SRS from each stratum
3. combine all the individuals chosen from all of the strata to make up the full sample
NOT a simple random sample/SRS
cluster sample
divides population into groups (or clusters), then randomly selects some of these clusters (completely ignoring the others). all of the individuals from the chosen clusters are selected to be in the sample
multistage sample design
selects successively smaller groups within the population in stages, resulting in a sample consisting of clusters of individuals. each stage may employ an SRS, a stratified sample, or another type of sample
systematic sample
inspecting every (ex. 25th) experimental unit
probability sample
gives each member of the population a known chance (greater than zero) to be selected
many types of this listed above
chosen by chance
undercoverage
occurs when some groups in the population are left out of the process of choosing the sample
the population is not the same group as the sampling frame (the group from which the sample is chosen)
nonresponse
occurs when an individual chosen for the sample cannot be contacted or refuses to cooperate
response bias
occurs when an individual in a sample chooses an answer to a survey that he/she thinks is best rather than the answer that he/she truly believes
wording of question
may influence a person by leading the individual being questioned to one answer as opposed to another
what need to know before trust a poll
exact questions asked, rate of nonresponse, and date and method of the survey
large or small
larger random samples give more accurate results than smaller samples
experimental units
individuals on which the experiment is being done
if units are people they are called subjects
treatment
specific experimental condition applied to the units
level of treatment measured by the explanatory variable and the level of the variable interested in is by by response variable
factors
explanatory variables in an experiment (changes being imposed on the subjects)
experiments may have several factors
three basic principles of experimental design
- control the effect of lurking variables on the response variable
- randomization ensures that individuals are assigned to treatment groups by chance
- replication is accomplished if each treatment is applied to multiple experimental units, ensures that results are not just due to chance variation
way to control for lurking variable
control group (second group involved, given placebo, treatment group and control group compared) or do comparison group, still called control
difference between control and control group
control: overall effort to minimize variability in the way the experimental units are obtained and treated
control group: a group that gets a placebo
randomized comparative experiment
an experiment that has random assignment and a control or comparison group
statistically significant
an observed effect too large to attribute plausibly to chance
placebo effect
occurs when a subject receiving a placebo reacts favorably to it (even though it isn’t actually doing anything)
block
a group of experimental units or subjects that are known before the experiment to be similar in some way that is expected to affect the response to the treatments
synonym = homogenous group
block design
random assignment of units to treatments carried out separately within each block
assignment of subjects to blocks is not random but assignment of subjects within each block to various groups is done randomly
saying
control what you can, block what you can’t, and randomize the rest
difference between blocking and randomizing
block to control variables you know about that might influence response
randomize to control for variables don’t know about
matched pairs design
type of block design
two possible set ups:
1. subjects are matched with themselves and given two different treatments in random order, more common
2. similar subjects are matched and each assigned a different treatment
blind experiment
subjects don’t know which treatment they’re receiving
double blind
neither the researchers nor the subjects know who is receiving which treatment
researcher v experimenter
researcher is person measuring the response variable
experimenter is person running overall experiment who knows who is receiving which treatment
lack of realism
most serious potential weakness of experiments
observational study v experiment
in observational study explanatory variable observed and measured, whereas in experiment, explanatory variable manipulated
design (of sampling)
the method used for selecting the sample from the population
steps to use random table of digits
- assign numerical label to every individual in the population
- use table B to select labels at random
- indicate when you should stop sampling
- use the labels to identify subjects selected to be in the sample
essential principle of statistical sampling
having a probability sample
levels
when factor has different degrees
frequency of recurrence
placebo
a pill that is made of inactive material which is used so that subjects can have information withheld about which treatment group they’re in
gold standard method of assuring the equivalence of two treatment groups
to randomly assign subjects to the two treatment groups to ensure they are assigned by chance
three central principles of experimental design
- control (making comparisons between groups)
- randomization (method of assigning individuals to groups)
- replication (how many individuals you have in your groups)
