Ark nr.: 1 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Rotation Tests - Computing exact adjusted p-values in multiresponse experiments Øyvind Langsrud, MATFORSK, Norwegian Food Research Institute.

Ark nr.: 2 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Campylobacter experiment  Three biological replicates (block variable)  312 FT-IR wavelengths as multiple responses  polysaccharide region [ cm -1 ]

Ark nr.: 3 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Analysis with MINITAB - first wavelength

Ark nr.: 4 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk ANALYSIS with MANOVA

Ark nr.: 5 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusted means as curves

Ark nr.: 6 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Effect of Day single response p-values  Ordinary significance tests are not longer suitable  A lot of significant results cased by random variation (since several tests/responses)  The p-values need to be adjusted  So that they are interpretable

Ark nr.: 7 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusted p-values  So that experimentwise (or familywise) error rate is controlled  Bonferroni correction (classical method)  pAdj = #responses pRaw  Conservative upper bound (in practice often too conservative )  Dependence among responses not investigated  Modern methods  Makes active use of dependence among responses  Permutation tests  Rotation tests

Ark nr.: 8 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Assume a regression model (simplified model without constant term)  Separate F-tests for each response  Random variables: F 1, F 2 …, F q  Observed values: f 1, f 2 …, f q  Maximal F-value (= minimum p-value) obtained for response number k  Raw p-value:  Adjusted p-value:

Ark nr.: 9 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusting the minimum p-value by permutations  For m =1,2 …. M  permute data ( Y  P (m) Y )  compute maximal F-statistic from these data  Compute p-value as

Ark nr.: 10 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk How is dependence handled?  Estimate of covariance matrix under H 0 :  Estimate based on permuted data:  The permutation test is a conditional test  Conditioned on the covariance matrix estimate  Conditioned on sufficient statistics for the unknown parameters  Fisher's exact test for 2  2 contingency tables  is the most famous conditional test

Ark nr.: 11 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Conditional test under multivariate normality?  Need distribution of Y conditioned on Y T Y  Answer  Y is distributed as RY obs  where Y obs is the observed matrix  and where R is an uniformly distributed orthogonal matrix (random rotation matrix)  Relation to well-known tests  t-test, F-tests, Hotelling T 2, Wilks’  are special cases of rotation testing  But these test statistics do not depend on Y T Y  Conditioning not needed  Simulations not necessary

Ark nr.: 12 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusting the minimum p-value by rotations  For m =1,2 …. M  simulate rotated data ( Y  R (m) Y )  where R (m) is a simulated random rotation matrix  compute maximal F-statistic from these data  Compute p-value as  In practice: a much more efficient algorithm is applied

Ark nr.: 13 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusting the other p-values (permutations or rotations)  Remove response with minimum p-value  Adjust minimum p-value in new data set  and so on  Enforce monotonicity  All calculations can be done simultaneously

Ark nr.: 14 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Permutation test or rotation test  Exact permutation testing  The only assumption: independent observations  Useless for few observations  Useless for complex ANOVA and regression models  Exact rotation testing  Assumes multivariate normality  Does not need as many observations as permutation testing  Can be use for complex ANOVA and regression models  F-test  rotation test

Ark nr.: 15 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusted p-values (FWE)  non-adjusted p-values (RAW)  False significance at 1% level is expected in 1% of all the investigated responses  If you have 5000 responses …..  “Classically ” adjusted p-values (FWE)  False significance at 1% level is expected in not more that 1% of all experiments where the method is applied.  The experimentwise (or familywise) error rate is controlled

Ark nr.: 16 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk False Discovery Rate (FDR)  Adjusted p-values according to False Discovery Rate  False significance at 1% level is expected in 1% of all cases (responses) reported as significant at 1% level.  If you have 5000 responses and 200 are reported as significant at 1% level, one will expect two of these as false.  “q-values” is proposed instead of “adjusted p-values”

Ark nr.: 17 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Calculation of FDR adjusted p-values  Several methods exist  Most of them do not handle the dependence among the responses  but OK if the “weak dependence requirement” is met  New variant  based on rotations (or alternatively permutations)  handles any kind of dependence  conservative compared to other methods  since the method does not involve an estimate of the amount of responses with true null hypotheses

Ark nr.: 18 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusted p-values (first 30 wavelengths)

Ark nr.: 19 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Adjusted p-values (30 most significant wavelengths)

Ark nr.: 20 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk p-values

Ark nr.: 21 | Forfatter: Øyvind Langsrud - a member of the Food Science Alliance | NLH - Matforsk - Akvaforsk Rotation Tests - Conclusion  Simulation principle for computing exact Monte Carlo p-value for any test statistic.  Based on multivariate normal distribution.  Generalisation of classical tests.  Related to permutation testing.  Useful for computing adjusted p-values (F-tests)  FWE, FDR  General linear models (ANOVA and regression)  Implemented in the MANOVA program ( )