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Group Discussion  Identify two time-related factors that can threaten the internal validity of a within- subjects research study. In each case, explain.

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Presentation on theme: "Group Discussion  Identify two time-related factors that can threaten the internal validity of a within- subjects research study. In each case, explain."— Presentation transcript:

1 Group Discussion  Identify two time-related factors that can threaten the internal validity of a within- subjects research study. In each case, explain how the factor could be a confounding variable.  Describe the process of counterbalancing and explain how this process reduces the risk of problems from order effects.

2 Chapter 9

3 Within & Between designs Within Subjects StudentsPhonics Whole Word A1215 B1314 C1514 D 15 E 14 Between Subjects StudentsPhonics Whole Word A12 B13 C15 D14 E15 F G 14 H I 15 J 14

4 repeated- measures  A within- subjects experimental design, also known as a repeated- measures experimental design, compares two or more different treatment conditions ( or compares a treatment and a control) by observing or measuring the same group of individuals in all of the treatment conditions being compared.

5 Advantages of Within- Subjects Designs 2 problems are reduced or eliminated in a within- subjects design. 1. Individual differences between groups can become a confounding variable. If the individuals in one treatment condition are noticeably different from the individuals in another treatment ( for example, smarter, faster, bigger, or older), the individual differences, rather than the treatments, may explain any observed differences. 2. The individual differences within each treatment condition can create high variance, which can obscure any differences between treatments.

6 Examples

7 Within subjects a within- subjects design is generally more powerful than a between- subjects design; that is, a within- subjects design is more likely to detect a treatment effect than a between- subjects design.

8 Threats to internal validity for within- subjects designs 1- Confounding from environmental variables (room difference, light difference, temperature difference)

9 2- Confounding from time- related factors. History. Maturation.. Instrumentation. Testing effects. Statistical Regression

10 3- participant attrition  Another potential problem for the within- subjects design is participant attrition.

11 4- Order Effects Carryover effect Contrast effect (lighting, boring teacher) Progressive error Practice effect Fatigue effect Reduced motivation effect

12 Dealing with time- related threats and order effects

13 1- Controlling Time if the different treatment conditions are scheduled over a period of months, the chances greatly increase that an outside event ( history, maturation, or change in the measurement instrument) will have an influence on the results. However if the time between treatments is too short other factors such as fatigue or reduced motivation may change the results

14 2- Switch to a Between- Subjects Design  In some situations, order effects are so strong and so obvious that a researcher probably would not even consider using a within- subjects design. For example, a within-subjects design is a poor choice for a study comparing two methods of teaching reading to first- grade children. After the children have been taught with method I, they are permanently changed.

15 3- Counterbalancing  The process of matching treatments with respect to time is called counterbalancing.

16 Easy Case  order effects evenly distributed between the treatment conditions  it doesn’t matter which treatment comes first  there is a constant (e.g., d=5 points) change due to order effect

17 Order effect Pop MusicClassic Music GenericBrand GenericBrand (27+5) 2032 (27+5) (29+5) 2334 (29+5) (29+5) 2534 (29+5) (26+5) 1931 (26+5) (31+5) 31 (25+5) (22+5) 22 (17+5) (20+5) 19(14+5) (24+5) 21 (16+5) No Order effect Counter Balanced

18 Limitations of Counterbalancing

19 Limitation 1 Counterbalancing balances the effect of ordering effect but it doesn’t eliminate it. So both means are inflated

20 Counterbalancing = =6 Pop music Classic MusicGenericBrand

21 Limitation 2  A more serious problem is when counterbalancing adds order effect to some of the individuals but not to all.

22 Limitation 3 When the order effect is not symmetrical One treatment might produce a larger order effect than the other treatment Example? (Math & Statistics) In such situations, the order effects are not symmetrical, and counterbalancing the order of treatments does not balance the order effects.

23 Limitation 4 Number of treatments  With only two treatment conditions, complete counterbalancing is easy: There are only two possible sequences. However, as the number of treatments increases, complete counterbalancing becomes more complex.

24 Number of Treatments

25 Partial counter-balancing  One solution to this problem is to use what is known as partial counter- balancing.  A simple and unbiased procedure for selecting sequences is to construct a Latin square.

26 Latin square

27 The Latin square is not a particularly good example of partial counterbalancing because it does not balance every possible sequence of treatment conditions. For example, the first three groups all receive treatment A followed immediately by treatment B. Limitation of Latin square

28 Random order  One method for improving the square is to use a random process to rearrange the columns ( for example, a coin toss to decide whether or not each column is moved)

29 Statistical analyses  With two treatment conditions, a repeated- measures t test  For more than 2 treatments a single-factor ANOVA ( repeated measures) can be used to evaluate the statistical significance of the mean difference.

30 Ordinal & nominal scale  If the data are measured on an ordinal scale ( or can be rank ordered), a Wilcoxon test can be used to evaluate significant differences.  If the data includes only positive and negative (nominal) effects then we use a sign test.

31 Matched- Subjects Designs  In a matched- subjects design, each individual in one group is matched with a participant in each of the other groups.  The goal of a matched- subjects design is to duplicate all the advantages of within- and between- subjects designs without the disadvantages of either one.

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