Ex Post Facto Experiment Design

Ahmad Alnafoosi

CSC 426 Week 6

Ex Post Facto what???

• Webster Dictionary defines Ex Post Facto as:• after the fact : retroactively

• Late Latin, literally, from a thing done afterward. First Known Use: 1621

Explain More…

• In situations where it is not possible to manipulate variables.

• Ex Post Facto design provides an alternative to investigate how independent variables affect dependant variables.

• The researcher can observe the independent variables after the event.

That sounds like Co-relational design?

• Co-relational design and Ex post facto design involve examining existing conditions.

• Ex Post Facto design has dependant and independent variables whereas Co-relational design does not.

What about experimental Design?

• Both experimental design and Ex post facto design have independent and dependant variables.

• Ex Post Facto differs that it does not introduce the presumed producing cause.

• Thus in Ex Post Facto the researcher is NOT able to draw firm cause and effect.

• Both share similar designs.

What does Ex Post Facto Design Look like

• Similar to Experimental design, ex post facto design has multiple forms.

• These form involve variation of events (experience), Observations, Groups and combination of the above.

Simple Ex Post Facto Design

Simple Ex Post Facto Design

• Similar to Static Group Comparison with the difference of the timing of the treatment (Experience).

• It is called Experience since the researcher can not control it.

• Association can be drawn from this study (NOT Cause and effect).

Factorial Design

• In designs that involve multiple dependant variables with Ex Post Facto design, Factorial design is needed.

Randomized Two Factor Design• 2 variables tested by 4

groups.

• Variable 1 effect can be studied by comparing group1 and group2 of that of group3 and group4.

• Variable 2 effect can be studied by comparing group 1 and group 3 of that of group 2 and group4

Randomized Two Factor Design - Cont

• This design is a generalized version of Solomon four group design. (event instead of experiment)

• This design allow to see the effect of each of the variables.

• It also can show the interaction effect of the variables.

Combined Experimental and Ex Post Facto Design

• Combining experiment with Ex Post Facto Experience

• It has Ex Post Facto component by initially selecting groups that have that experience.

• Then there is experimental phase where where experiment is conducted.

Combined Experimental and Ex Post Facto Design - Cont

• The results will be 4 groups all possible combinations of experience and experiment.

• This design enables the study of experiment effect the dependant variables

• Also it enables the study of how previous experience interact with the experiment.

Sampling

Ahmad Alnafoosi

CSC 426 Week 6

Choosing a Sample in Descriptive Study

• The purpose of descriptive study is to be able to determine and describe large population.

• In most instances surveying all the population is not possible because of the sheer size.

• On the other hand the sample needs to be large enough to be representative of the population and their characterizations that are relevant to the study.

Sampling Design

• To achieve the aforementioned goals a sampling design is needed.

• The sampling design needs to take into consideration the actual traits of the population to apply the appropriate sampling design.

Probability Sampling

• Researcher can specify that each segment of the population will be represented in the sample.

• The sample is chosen using Random Selection (each member of the population has equal chance to be picked)

Simple Random Sampling

• Is a probability sampling design.

• Each member has equal chance to be picked.

• Used for small population where every member is know.

Stratified Random Sampling

• Is a probability Sampling design.

• Is used in stratified population where there is multiple layers strata

• Guarantee that each of the identified strata.

• Is used when the stratum are equal in size.

Proportional Stratified Sampling

• is probability sampling design.

• When the population is stratified but where stratum are not equal in size.

• In this case the number of random sample of each strata taken is dependant proportionally to the strata population to the whole population.

Cluster Sample

• is probability sampling design.

• Is used when the population is spread over large area.

• Clusters need to be similar to each other as much as possible.

• Each cluster has to have equal heterogeneous population.

Systematic Sampling

• Is probability sampling design.

• Involve selecting individuals based on pre-determined sequence.

• The sequence needs to be random.

Factors in determining Probability Sample Design

• Population size

• Stratification

• Size of stratum

• Clustering

Non-Probability Sampling

• Does not guarantee that each element of the population will be represented in the sample.

• Some members of the population have no chance of being represented.

Convenience Sampling

• Is non probability sampling design.

• AKA Accidental sampling.

• It sample available members of the population.

Quota Sampling

• Is Non-probability Sampling

• It select individuals in the same proportion as they are found in the general population, but it is not random.

Purposive Sampling

• Is Non-probability Sampling

• It select individuals for a particular purpose.

• Needs to be careful since it assume that the chosen sample is useful for the purpose.

Sampling Surveys of very large population

• To tackle very large population multi-staging of sampling areas might be needed.

• This involves• Primary area selection• Sample location selection• Chunk selection• Segment selection• Housing selection

What is the right sample size

• For population less than 100, sample the entire population.

• For population around 500, sample 50%

• For population around 1,500 , sample 20%

• For population larger than 5,000 sample size can be around 400.

Sample Bias

• Sampling will introduce bias into the sample.

• Researcher need to acknowledge.