Differential Expression for RNA-Seq

The Differential Expression for RNA-Seq tool performs a statistical differential expression test for a set of Expression Tracks. The statistical analysis is described in more detail in The statistical model.

To run the Differential Expression for RNA-Seq analysis:

        Toolbox | Transcriptomics Analysis (Image expressionfolder)| RNA-Seq Analysis | Differential Expression for RNA-Seq

Select a number of Expression tracks (Image rnaseqtrack_16_h_p) and click Next.

Image experimental_design
Figure 1.5: Setting up the experimental design and comparisons

This will display the wizard shown in figure 1.5.

In the Experimental design panel, a Metadata table must be selected that describes the factors and groups for all the samples.

The Comparisons panel determines the number and type of statistical comparison tracks output by the tool (see IOutput of the Differential Expression for RNA-Seq tool for more details).

How many replicates do I need? The Differential Expression for RNA-Seq tool is capable of running without replicates, but this is not recommended and the results should be treated with caution. In general it is desirable to have as many biological replicates as possible - typically at least $ 3$. Replication is important in that it allows the `within group' variation to be accurately estimated for a gene. In the absence of replication, the Differential Expression for RNA-Seq tool assumes that genes with similar average expression levels have similar variability.

Technical or biological replicates? [Auer and Doerge, 2010] illustrates the importance of biological replicates with the example of an alien visiting Earth. The alien wishes to know if men are taller than women. It abducts one man and one woman, and measures their heights several times i.e. performs several technical replicates. However, in the absence of biological replicates, the alien would erroneously conclude that women are taller than men if this was the case in the two abducted individuals.


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