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Sample A: Cover Page of Thesis, Project, or Dissertation Proposal

Sample A: Cover Page of Thesis, Project, or Dissertation Proposal

Sample A: Cover Page of Thesis, Project, or Dissertation Proposal

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Probe Cleansing Methods<br />

The BaFL protocol (Chapter 2) was applied to the datasets and the mean <strong>of</strong> the cleansed probe<br />

values was computed to obtain a scalar f<strong>or</strong> the remaining ProbeSets in each sample; as bef<strong>or</strong>e we<br />

required that there be 4 <strong>or</strong> m<strong>or</strong>e probes in a ProbeSet that were common to all samples in the<br />

class; here we look at a two-class problem. RMA and dCHIP implementations in R-affy [15,<br />

16] were used to generate ProbeSet values f<strong>or</strong> the same set <strong>of</strong> samples. Li and Wong proposed<br />

dCHIP as the implementation <strong>of</strong> a model based expression index (MBEI) [2, 17]. The heart <strong>of</strong><br />

their alg<strong>or</strong>ithm utilizes a weighted average <strong>of</strong> mismatch differences:<br />

~<br />

$ " ( # ) / J , (1)<br />

i<br />

!<br />

yi j j<br />

with i representing the samples, J representing the number <strong>of</strong> probes f<strong>or</strong> a probeset yij and # j the<br />

probe level mismatch difference. The weighting scheme fav<strong>or</strong>s probes with the largest PM/MM<br />

difference. The w<strong>or</strong>kh<strong>or</strong>se f<strong>or</strong> this alg<strong>or</strong>ithm is the probe sensitivity index ( # j ), which identifies<br />

probes with large standard err<strong>or</strong> and negativity (MM > PM) [2, 17]; the likely sources <strong>of</strong> this<br />

effect are cross hybridization and lab<strong>or</strong>at<strong>or</strong>y handling. However, these sources are going to be<br />

inconsistent across experiments. Their publication makes the claim that the probe sensitivity<br />

index should be independent <strong>of</strong> the tissue type [17]. One can readily give a counter-example to<br />

the assumption <strong>of</strong> this statement: given a probe that has two potential transcript is<strong>of</strong><strong>or</strong>ms to which<br />

it can hybridize and 3 tissues, if tissue A has little expression f<strong>or</strong> both transcripts, tissue B has<br />

expression <strong>of</strong> only one transcript and tissue C expresses both transcripts well, then the probe<br />

sensitivity index will reduce the contribution <strong>of</strong> the probe consistently only f<strong>or</strong> tissue C. RMA<br />

implements an additive model and considers only the PM data, after perf<strong>or</strong>ming a background<br />

69

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