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# Experimental+designs

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### Experimental+designs

1. 1. Experimental Designs
2. 2. • Laboratory experiment- researcher creates a situation with the desired condition and then manipulates some variables while controlling others. • A field experiment is a study in a realistic or natural setting and involves manipulation of the independent variable while carefully controlling the others
3. 3. Randomized designs • Completely Randomized Designs (CRD)- an experimental design that uses a random process to assign experimental units to treatments • A Randomized Block Design- Grouping test units into homogenous blocks (similar to stratified sampling)
4. 4. Validity • Internal validity: Is the effect due to our hypothesized cause? • External validity: Can we generalize to other samples?
5. 5. Experimental designs • X refers to the exposure of the subject to an experimental treatment • O refers to the process of observation and measurement of the test units Eg: X O1 O2
6. 6. Extraneous variables • History- specific events external to the experiment but occurring at the same time , that may affect the criterion variable • Maturation-Changes occurring within test units that are not due to the experimental variable but due to the passage of time • Testing-the experiment itself may affect the responses
7. 7. • Instrument variation- changes in the instrument may account for the differences • Statistical regression-the tendency of the extreme cases to move closer to the average during the course of the experiment • Selection bias- the way in which the test units are selected
8. 8. Pre experimental design • One shot case study: X O • One group pretest-post test design: O1 X O2 • Static group comparisonEG X O1 CG O2 No way of ensuring the two groups are equivalent prior to the comparison
9. 9. True experimental designs • Before-After with control group design EG: (R) O1 X O2 CG: (R ) O3 O4 The difference between O3 and O4 reflects external causes. But they would exert a similar influence on EG also O2-O1= E+U O4-O3= U Design is nearly ideal. Accounts for all external causes except Interactive testing. So strictly: O2-O1=E+U+I O4-O3= U
10. 10. Four group- six study design • • • • EG 1: CG 1: EG 2: CG 2: O1 X O2 O2-O1 = E+U+I O3 O4 O4-O3= U X O5 O5- ½ (O1+O3)= E+U O6 O6- ½ (O1+O3)= U
11. 11. Time series experimental designs • Several observations taken before the treatment; then the treatment (X) is administered, then several observations are taken to measure if there is a change in the pattern after as compared to before the experiment • O1 O2 O3 X O4 O5 O6
12. 12. Factorial designs • Allow for the testing of two or more treatments (factors) at various levels. • Main effect: the influence on a dependant variable by each independent variable separately • Interactive effect: the effect on a dependant variable by combinations of two or more independent variables • Interactive effects may be more than the total of the main effects
13. 13. • A researcher wants to understand the influence of both gender and the advertisement (2 advts are beingconsidered) on ‘believability’. There are 2 independent variables: gender and advt. • Each variable has 2 levels (2 advts and 2 genders)- leads to a 2X2 factorial design
14. 14. Latin square design • Attempts to control or block out the confounding effect of extraneous factors. • A balanced two-way classification scheme • In a Latin square design, both the confounding factors must have the same number of levels • Eg: A taste test involving 3 levels of tastes administered to 3 different people with the order of the administration randomized