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Sample Size Determination for Efficient use of Resources PGRM 3.5

Sample Size Determination for Efficient use of Resources PGRM 3.5. 4 determining factors: A – D . Variability of experimental material Expressed as either (i) the Standard deviation ( SD ) of the response

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Sample Size Determination for Efficient use of Resources PGRM 3.5

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  1. Sample Size DeterminationforEfficient use of ResourcesPGRM 3.5

  2. 4 determining factors: A – D Variability of experimental materialExpressed as either (i) the Standard deviation (SD) of the response (ii) the CV (= 100*SD/Mean).The CV for biological responses is often in range 10-30%. Size of difference expected (d)Based on (i) Knowledge of similar work. (ii) Knowledge of the science (iii) economically important difference? A B

  3. Estimating the SD & CV (for A) From analysis of similar data: SD is estimated by √MSE From the literature: SEM (SE of mean) = √(MSE/r)so SD = SEM × √r Example:Chowdhury and Rosario (1994)J. Agric. Sci. Camb 122, 193-199 Randomised block with 5 blocks (r=5) SEM for Dry matter yield = 0.206SD estimated by 0.206 × 5 = 0.460 Mean ≈ 5 so CV≈ 100 × 0.460/5 = 9.2%

  4. Estimatingthe SD & CV (contd) Example:Wayne el al. (1999).J. Ecol 87,183-192 Replication = 6 SED for reproductive weight per stand is 0.295. Recall! SED = √2 × SEM so SD = √(r/2) × SED SD estimated by 0.295 × √(6/2) = 0.51 Mean ≈ 1.2 CV ≈ 100 × 0.51/1.2 = 42.5%

  5. More determining factors: C & D C Criteria for rejecting the null hypothesisSignificance Level = Probability of rejecting the null when it is true.(ie concluding there is a difference when there is not)Recall: rejecting when p < 0.05 gives significance level 0.05Typical levels : 0.05, 0.01, 0.001 Power = Probability of concluding there is a difference when there is one of size dTypical levels: 0.8. 0.9, 0.95 D

  6. Calculation of replicates per treatment Fixing significance at 0.05, and power at 80% To detect a d% difference the required replication per treatment is: r = 16(CV/d)2 ExampleCV = 15%, d = 10%,r = 16 (15/10)2= 36

  7. Review of Resource Use To see how precise an experiment actually was the formula above can be rewritten as d = 4 CV/r to give d = 4 SEM% (= 2.82 SED%) where SEM (SED) are expressed as % of the overall mean. Example follows:

  8. Example: Review of resource use Suppose an experiment with two treatments has the following result Treatment 1 2 SEM 10.7 13.9 1.1 The grand mean is 12.3 and the SEM as a percentage of that is 8.9%.The formula says that a real underlying difference between treatments of size 4 x 8.9% = 35.6% would have about an 80% chance of being detected at the 5% level in this experiment.

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