What comprises each of the following three: observational studies, designed experiments, and sampling methods
(1) Observational Studies (end with 'studies')
- these are retrospective
. Reserachers look back in time at existing records. In these studies, individuals with certain characteristics are matched with those who do not have these characteristics.
: better than cross-sectional
: memory of participants may not be accurate and records might be lost.
- Cross-Sectional Studies - studies that collect
- information about individuals at a specific point in time or over a very short period.
- called prospective
studies because they are done going forward. A group of individuals (cohort) is identified to participate and is observed over time (characteristics are recorded over time by the researcher)
: no need to rely on cohort to report info to researchers
: time and labor intensive
*Cross-sectional and case control are relatively inexpensive and allow researchers to explore possible associations before undertaking large cohort studies or designed experiments.
Remember there is no point in spending energy obtaining data that already exists.
(2) Designed Experiments (end with 'design')
Completely Randomized Design
- a study wherein each subject is randomly assigned to a treatment.
- (a.k.a. before-after or pretest-posttest experiments)
This is an experimental design in which the subjects are paired up. Pairs
are selected so that they are related in some way
(twins, siblings, husband and wife, same geographical location, etc). There are only two levels of treatment
in a matched-pairs design. One individual receives treatment, and the other receives the other treatment. Treatment is assigned by randomization
. Often, the response variable is measured on each subject before and after treatment has been applied matching the individual against itself. In doing this, the individual is matched against him/herself.
(3) Sampling methods (end with 'sampling')
Simple Random Sampling
- a sample of size "n" from a population size "N". it is obtained if every possible sample of size "n" has an equal chance of occurring. sample size is always less than the size of the population.
- by dividing the population into non-overlapping groups called strata
and then obtaining a simple random sample from each stratum
. The individuals in each strata should be similar in some way (homogenous).
: may allow fewer to be surveyed while obtaining the same or more information
- No frame is required
obtained by selecting every "kth
" individual from the population. The first individual selected corresponds to a number between 1 and "k" (e.g., every 10th individual). if the size of the population is unknown, there is no way to mathematically find "k"
- pros: less chance of interviewer error, easier to employ, typically provides more info for a given cost than does Simple Random sampling (SRS)
- Cluster Sampling - obtained by selecting all individuals within a randomly selected collection or group of individuals.
- pros: reduce travel time likely required in stratified or SRS, no need to obtain a frame
- Convenience Sampling - (a.k.a. voluntary response samples) the individuals are easily obtained and not based on randomness. most popular convenience samples are those in which the individuals are self-selected