Tutorial: Normalize datasets
TRANSIT has the capability to normalize datasets with different methods, and export them to IGV from the Broad Institute or a CombinedWig format. This tutorial shows a quick overview of how to normalize datasets save them using the GUI mode of transit or through the Console mode.
Adding the annotation file
Before we can normalize .wig datasets, we need to add an annotation file for the organism. Click on the file dialog button, on the top of the TRANSIT window (see image below), and browse and select the appropriate annotation file. Note: Annotation files must be in “.prot_table” or GFF3 format, described above:
Add .wig datasets
Next we must choose to add .wig formatted datasets what we wish to normalize to CombinedWig format. To add these, we click on the control sample file dialog (see image below), and select the desired datasets (one by one). In this example, we have two replicates:
As we add the datasets they will appear in the table below. Select the datasets you wish to normalize.
Normalize and Save
After you have selected the desired datasets in the list of datasets added, click on “Export -> Selected Datasets” in the menu bar at the top of the TRANSIT window, and select the format you desire (e.g. “to IGV” or “to CombinedWig”). You will be prompted to pick a normalization method, and a filename. Note: Only selected datasets (“Control+Click”) will be normalized and saved.
Proper normalization is important as it ensures that other sources of variability are not mistakenly treated as real differences in datasets. TRANSIT provides various normalization methods, which are briefly described below:
Trimmed Total Reads (TTR), normalized by the total read-counts (like totreads), but trims top and bottom 5% of read-counts. This is the recommended normalization method for most cases as it has the beneffit of normalizing for difference in saturation in the context of resampling.
Normalizes datasets to have the same mean over the non-zero sites.
Normalizes datasets by total read-counts, and scales them to have the same mean over all counts.
Fits a zero-inflated negative binomial model, and then divides read-counts by the mean. The zero-inflated negative binomial model will treat some empty sites as belonging to the “true” negative binomial distribution responsible for read-counts while treating the others as “essential” (and thus not influencing its parameters).
Normalizes datasets using the quantile normalization method described by Bolstad et al. (2003). In this normalization procedure, datasets are sorted, an empirical distribution is estimated as the mean across the sorted datasets at each site, and then the original (unsorted) datasets are assigned values from the empirical distribution based on their quantiles.
Normalizes the datasets to fit an “ideal” Geometric distribution with a variable probability parameter p. Specially useful for datasets that contain a large skew.
No normalization is performed.