Two Methods for Collecting Data
Observational Studies
Designed Experiments
Explanatory + Response Variable
Changing the amount of an "explanatory variable" affects the value of a "response variable.
Explanatory Variable Example
I punch you.
Response Variable Example
How bad will it hurt?
Observational Study
(An observational study measures the value of response variable without attempting to influence any of the response or explanatory variables of the individual).
That is, in an observational study, the researcher observes the behavior of the individuals in
Designed Experiment
(In Designed Experiments, individuals are assigned to groups where explanatory variables are intentionally changed and the values of the response variables are rewarded).
If a researcher assigns the individuals in a study to a certain group, intentionally
Confounding
(Confounding occurs in a study when you are unable to distinguish among the effects of two or more explanatory variables.)
Therefore, any relation that may exist between the explanatory variable and lurking variable may be due to some other variable or va
Lurking Variable
A lurking variable is an explanatory variable that was not considered in a study, but that affects the value of the response variable in the study. In addition, lurking variables typically relate to explanatory variables considered in the study. (Remember
Observational Studies Remark
Observational studies do not allow a researcher to claim causation, only association. You just can't say "smoking causes lung cancer.
Designed Experiments Remark
Are used whenever control of certain variables is possible and desirable.
You wouldn't want to do a design experiment having people smoke cigs. That is why Observational Studies are necessary. They also have lower cost, greater timeliness, and broader ran
Three Categories of Observational Studies
Cross-sectional studies
Case-control studies
Cohort studies
Cross-Sectional Studies
Observational studies that collect information about individuals at a specific point in time (all at once) or over a very short period of time.
Case-Control Studies
These studies are retrospective, meaning that they require individuals to look back in time or require the researcher to look at existing records. In case-control studies, individuals that have a certain characteristic are matched with those that do now.
Cohort (Prospective) Studies
A cohort study first identifies of a group of individuals to participate in the study (the cohort). The cohort is then observed over a period of time (sometimes a long period of time). Over this time period, characteristics about the individuals are recor
Cohort (Prospective) Studies 2
They require many individuals to participate over long periods of times. A disadvantage is that individuals tend to drop out due to the long time frame. They are the most powerful in Cohort studies.
Census
(The collection of data from every member of a population is called a census).
US decennial census.