Problems in Data Analyses Mostafa

1. Problems in Data AnalysesMostafa

2. General case in data analysis • Assumptions distortion • Missing data

3. General Assumption of Anova • The error terms are randomly, and normally distributed Populations (for each condition) are Normally Distributed • The variance of different population are homogeneous (Homo-scedasticity) Populations (for each condition) have Equal Variances • Variances and means of different populations are not correlated (independent)

4. CRD ANOVA F-Test Assumptions • Randomness & Normality • Homogeneity of Variance • Independence of Errors

5. Randomized Block F Test Assumptions 1. Normality Populations are normally distributed 2. Homogeneity of Variance Populations have equal variances 3. Independence of Errors Independent random samples are drawn 4. No Interaction Between Blocks & Treatments

6. Diagnosis: Normality • The points on the normality plot must more or less follow a line to claim “normal distributed”. • There are statistic tests to verify it scientifically. • The ANOVA method we learn here is not sensitive to the normality assumption. That is, a mild departure from the normal distribution will not change our conclusions much. Normality plot: normal scores vs. residuals

7. Normality Tests • Wide variety of tests can be performed to test if the data follows a normal distribution. • Mardia (1980) provides an extensive list for both the uni-variate and multivariate cases and it is categorized into two types: • Properties of normal distribution, more specifically, the first four moments of the normal distribution • Shapiro-Wilk’s W (compares the ratio of the standard deviation to the variance multiplied by a constant to one) • Lilliefors-Kolmogorov-Smirnov Test • Graphical methods based on residual error (Residual Plotts) • Goodness-of-fit tests, • Kolmogorov-Smirnov D • Cramer-von Mises W2 • Anderson-Darling A2

8. Formal Tests of Normality • Kolmogorov-Smirnov test; Anderson-Darling test (both based on the empirical CDF). • Shapiro-Wilk’s test; Ryan-Joiner test (both are correlation based tests applicable for n < 50). • D’Agostino’s test (n>=50). All quite conservative – they fail to reject the null hypothesis of normality more often than they should.

9. The Consequences of Non-Normality • F-test is very robust against non-normal data, especially in a fixed-effects model • Large sample size will approximate normality by Central Limit Theorem (recommended sample size > 50) • Simulations have shown unequal sample sizes between treatment groups magnify any departure from normality • A large deviation from normality leads to hypothesis test conclusions that are too liberal and a decrease in power and efficiency

10. Remedial Measures for Non-Normality • Data transformation • Be aware - transformations may lead to a fundamental change in the relationship between the dependent and the independent variable and is not always recommended. • Don’t use the standard F-test. • Modified F-tests • Adjust the degrees of freedom • Rank F-test (capitalizes the F-tests robustness) • Randomization test on the F-ratio • Other non-parametric test if distribution is unknown • Make up our own test using a likelihood ratio if distribution is known

11. Homogeneity of Variances • Eisenhart (1947) describes the problem of unequal variances as follows • the ANOVA model is based on the proportion of the mean squares of the factors and the residual mean squares • The residual mean square is the unbiased estimator of 2, the variance of a single observation • The between treatment mean squares takes into account not only the differences between observations, 2,just like the residual mean squares, but also the variance between treatments • If there was non-constant variance among treatments, the residual mean square can be replaced with some overall variance,  a2, and a treatment variance,  t2, which is some weighted version of  a2 • The “neatness” of ANOVA is lost

12. Homogeneity of Variances • The overall F-test is very robust against heterogeneity of variances, especially with fixed effects and equal sample sizes. • Tests for treatment differences like t-tests and contrasts are severely affected, resulting in inferences that may be too liberal or conservative • Unequal variances can have a marked effect on the level of the test, especially if smaller sample sizes are associated with groups having larger variances • Unequal variances will lead to bias conclusion

13. Tests for Homogeneity of Variances • Bartley’s Test • Levene’s Test Computes a one-way-anova on the absolute value (or sometimes the square) of the residuals, |yij – ŷi| with t-1, N – t degrees of freedom Considered robust to departures of normality, but too conservative • Brown-Forsythe Test A slight modification of Levene’s test, where the median is substituted for the mean (Kuehl (2000) refers to it as the Levene (med) Test) • The Fmax Test (Hartley Test) Proportion of the largest variance of the treatment groups to the smallest and compares it to a critical value table

14. Levene’s Test More work but powerful result. = sample median of i-th group Let df1 = t -1 df2 = nT - t Reject H0 if Essentially an Anova on the zij

15. Independence It is a special case and the most common cause of heterogeneity of variance • Independent observations • No correlation between error terms • No correlation between independent variables and error • Positively correlated data inflates standard error • The estimation of the treatment means are more accurate than the standard error shows.

16. Independence Tests • If some notion of how the data was collected is understandable, check can be done if there exists any autocorrelation. • The Durbin-Watson statistic looks at the correlation of each value and the value before it • Data must be sorted in correct order for meaningful results • For example, samples collected at the same time would be ordered by time if suspect results could be depent on time

17. Independence • A positive correlation between means and variances is often encountered when there is a wide range of sample means • Data that often show a relation between variances and means are data based on counts and data consisting of proportion or percentages • Transformation data can frequently solve the problems

18. Remedial Measures for Dependent Data • First defense against dependent data is proper study design and randomization • Designs could be implemented that takes correlation into account, e.g., crossover design • Look for environmental factors unaccounted for • Add covariates to the model if they are causing correlation, e.g., quantified learning curves • If no underlying factors can be found attributed to the autocorrelation • Use a different model, e.g., random effects model • Transform the independent variables using the correlation coefficient

19. Missing data Observations that intended to be made but did not make. Reason of missing data: • An animal may die • An experimental plot may be flooded out • A worker may be ill and not turn up on the job • A jar of jelly may be dropped on the floor • The recorded data may be lost Since most experiment are designed with at least some degree of balance/symmetry, any missing observations will destroy the balance

20. Missing data • In the presence of missing data, the research goal remains making inferences that apply to the population targeted by the complete sample - i.e. the goal remains what it was if we had seen the complete data. • However, both making inferences and performing the analysis are now more complex. • Making assumptions in order to draw inferences, and then use an appropriate computational approach for the analysis is required • Consider the causes and pattern of the missing data for making appropriate changes in the planned analysis of the data

21. Missing data • Avoid adopting computationally simple solutions (such as just analyzing complete data or carrying forward the last observation in a longitudinal study) which generally lead to misleading inferences. • In one factor experiment, the data analysis can be executed with good estimated value, but in the factorial experiment theoretically can not be analyzed • In CRD one factor experiment, if there are missing data, data can be analyzed with different replication numbers • In the RCBD one factor experiment, if 1 – 2 complete block or treatment is missing but there are still 2 blocks complete, data analysis simply can be proceeded

22. Missing data • In the RCBD/LS one factor is experiment, if there 1 – 2 missing observations in the block or treatment , data can be treated by : a. the appropriate method of unequal frequencies b. the use of estimating unknown value from the observed data • The estimate of the missing observation most frequently is the value that minimizes the experimental error sum of square when the regular analysis is performed

23. Imputation • The error df should be reduced by one, since M was estimated • SAS can compute the F statistic, but the p-value will have to be computed separately • The method is efficient only when a couple cells are missing • The usual Type III analysis is available, but be careful of interpretation • Little and Rubin use MLE and simulation-based approaches • PROC MI in SAS v9 implements Little and Rubin approaches