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**Dr. AJIT SAHAI Director – Professor Biometrics JIPMER, Pondicherry**

ANOVA Dr. AJIT SAHAI Director – Professor Biometrics JIPMER, Pondicherry

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Analysis of Variance (ANOVA) Is a technique whereby the total variation present in a data set is partitioned or segregated into several components Usually each of these components of variation is associated with a specific source of variation In any experimentation it is of interest to know the magnitude of the contributions of each of these sources to the total variation

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**Hypotheses We may test the. null hypothesis that,**

Hypotheses We may test the null hypothesis that, - all population or treatment means are equal against the alternative that, - the members of at least one pair are not equal

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**- In situations warranting multiple comparisons. of means**

- In situations warranting multiple comparisons of means -a global test like ANOVA is desirable - mainly to examine whether there are any differences in the data; prior to testing various combinations of means to determine individual group differences

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**If a global test is not performed. - multiple tests between different**

If a global test is not performed - multiple tests between different pairs of means will alter the alpha level, not for each comparison but for the experimentation as a whole

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For example, if four drug levels with their six possible combinations are to be compared, and each comparison is made by using Alpha = .05, -there is a 5% chance that each comparison will falsely be called significant;

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**i. e. a type I error may occur six times;**

i.e. a type I error may occur six times; Overall, therefore, there is a 30% chance (6 * 5%) of declaring one of the comparisons incorrectly significant. Of course this 30% is only an approximation; it does not mean that all the comparisons are not independent.

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**So the recommended use of ANOVA protects the researcher against**

So the recommended use of ANOVA protects the researcher against error inflation - by first asking if there are differences at all among means of the groups.

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Some basic concepts in experimental designs are the minimum requirements to appreciate the approach of ANOVA in estimating and testing the hypotheses about - population means or about - population variances.

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It may be pointed out that when experiments are designed with the analysis in mind, researchers can, before conducting experiments, identify those sources of variation that they consider important and choose a design that will allow them to measure the extent of the contribution of these sources to total variation.

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The Completely Randomized Design (CRD) and the Randomized Complete Block Designs (RCBD) - are commonly used in Pharmacological experimentations, requiring the application of One-way and - Two-way Analyses of Variance, respectively.

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**In case the crucial assumptions**

In case the crucial assumptions** of ANOVA are not met, one may wish to consider a parallel non-parametric test such as - Kruskal – Wallis procedure or - Friedman procedure, respectively,for One or Two-way ANOVA

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**Assumptions. - Observed data constitute independent**

Assumptions - Observed data constitute independent random sample from the respective populations. - Each of the populations from which the sample comes is normally distributed. - Each of the populations has the same variance.

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Tests of significance The frequently used tests with there suitability to the situations are, - t-test for the paired or independent groups (with or without adjusting the level downward); - Bonferroni’s t-method or - Dunn’s multiple-comparison procedure; - Posteriori, or Post Hoc, comparisons; Contd.

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**Tukey’s HSD (Honestly Significant Difference)**

Tukey’s HSD (Honestly Significant Difference) - Scheffe’s Procedure; - Newman-Keuls Procedure; - Dunnett’s Procedure; - Duncan’s new multiple-range test and - Least Significant Difference (LSD)

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1 1 Slide © 2005 Thomson/South-Western Chapter 13, Part A Analysis of Variance and Experimental Design n Introduction to Analysis of Variance n Analysis.

1 1 Slide © 2005 Thomson/South-Western Chapter 13, Part A Analysis of Variance and Experimental Design n Introduction to Analysis of Variance n Analysis.

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