Week 07 Notes_1 - Week 7 Randomized Block Fixed Model...

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Week 7 Randomized Block – Fixed Model Example: Consider three treatments such as three teaching methods or three marketing techniques. Using a Single Anova technique, H 0 assumes that all means are equal μ 1 = μ 2 = μ 3 Let us assume that the null is true as the table below indicates. However, if the sum of the e ij are ‘quite’ negative in treatment 1 and ‘quite’ positive in the second treatment, this would separate the sample means, and H 0 might be falsely rejected (a type one error). This could happen if the poorer students in general happened to be assigned to treatment 1, and the better students to treatment two. Treatment 1 Treatment 2 Treatment 3 Pop. Means 30 30 30 Error ← - → + What would increase the probability that the sum or the average of the error terms would deviate meaningfully from zero. Remember assignments are at random. Small sample sizes would increase the likelihood of that happening. For example, the probability of five positive error terms in a row is 1 out of 32. differs from μ 1 by the average of the e i1 Imagine another scenario. Imagine that the Null hypothesis of equality of mean is false as the following table would indicate. If the sum of the e ij are ‘quite’ positive in treatment 1 and negative in the second treatment, this would bring the sample means together, and H 0 might be falsely accepted (a type two error). This could happen if the better students in general happened to be assigned to treatment 1, and the poorer students assigned to treatment two. Treatment 1 Treatment 2 Treatment 3 Pop. Means 20 30 40 Error → + ← - The next technique that we are about to study is superior to one way Analysis of Variance in that it reduces the probability that Individuals assigned to the treatments 1
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have a positive or negative bias. And, if successful, it greatly increases the ability to discover significant differences, by increasing power. This is accomplished by reducing the variance attributed to error variance. X ij = μ + α i + β j + e ij α i (block effect) is a new use of α To apply, consider three teaching effects with 15 students that first take a quantitative test before being assigned to a treatment. The students with the 3 lowest scores on the quantitative test are assigned to the three different treatments randomly. This is the first block (of students). One would think that this first block effect would be highly negative.
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This note was uploaded on 08/24/2010 for the course MATH 267 taught by Professor Chandrasekhar during the Summer '10 term at Anna University Chennai - Regional Office, Coimbatore.

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Week 07 Notes_1 - Week 7 Randomized Block Fixed Model...

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