Studied by 3 people
Observational study
observe outcomes without imposing any treatment
Experiment
Actively impose some treatment in order to observe the response
Experimental unit
the single individual (person, animal, plant, etc.) to which the different treatments are assigned
Factor
is the explanatory/independent variable that you change
Level
a specific value for the factor
Response variable
what you measure
Treatment
a specific experimental condition applied to the units
Control group
a group that is used to compare the factor against; can be a placebo or the “old” or current item
Placebo
a “dummy” treatment that can have no physical effect
blinding
method used so that units do not know which treatment they are getting
double blind
neither the units nor the evaluator know which treatment a subject received
Principles of Experimental Design
Control of effects of extraneous variables on the response – by comparing treatment groups to a control group (placebo or “old”)
Replication of the experiment on many subjects to quantify the natural variation in the experiment
Randomization – the use of chance to assign subjects to treatments
How do you show cause-and-effect
The ONLY way to show cause & effect is with a well-designed, well-controlled experiment!!!
Completely randomized –
Experimental design
all experimental units are allocated at random among all treatments
Randomized block
units are blocked into groups (homogeneous) and then randomly assigned to treatments
Experimental design
Based on characteristics that may affect results, not the treatments themselves
Matched pairs
a special type of block design
Experimental design
match up experimental units according to similar characteristics & randomly assign on to one treatment & the other automatically gets the 2nd treatment
have each unit do both treatments in random order
the assignment of treatments is dependent
Confounding variable
the effect of the confounding variable on the response cannot be separated from the effects of the explanatory variable (factor)
How do we gather data?
Surveys
Opinion polls
Interviews
Studies
Observational
Retrospective (past)
Prospective (future)
Experiments
Population
the entire group of individuals that we want information about
Census
a complete count of the population
Sample
A part of the population that we actually examine in order to gather information
Use sample to generalize to population
Sampling design
refers to the method used to choose the sample from the population
Sampling frame
a list of every individual in the population
Simple Random Sample (SRS)
Sampling design
consist of n individuals from the population chosen in such a way that
every individual has an equal chance of being selected
every set (group) of n individuals has an equal chance of being selected
Stratified random sample
Sampling design
population is divided into homogeneous groups called strata
SRS’s are pulled from each strata
Systematic random sample
Sampling design
select sample by following a systematic approach
randomly select where to begin
Cluster Sample
Sampling design
based upon location
randomly pick a location & sample all there
Multistage sample
Sampling design
select successively smaller groups within the population in stages
SRS used at each stage
Random digit table
Each entry is equally likely to be any of the 10 digits
digits are independent of each other
Numbers can be read across, diaganolly, vertically
Bias
A systematic error in measuring the estimate
favors certain outcomes
Sources of Bias
things that can cause bias in your sample
cannot do anything with bad data
Voluntary response
People chose to respond
Usually only people with very strong opinions respond
Convenience sampling
Ask people who are easy to ask
Produces bias results
Undercoverage
some groups of population are left out of the sampling process
Nonresponse
occurs when an individual chosen for the sample can’t be contacted or refuses to cooperate
telephone surveys 70% nonresponse
Response bias
occurs when the behavior of respondent or interviewer causes bias in the sample
wrong answers
Wording of the Questions
wording can influence the answers that are given
connotation of words
use of “big” words or technical words
