Normalization

# Background Correction (Oligonucleotide arrays)
- The array is split into 16 rectangular zones
- Zone background is chosen to be the lowest 2% of intensities in each zone
- The background for each of the probes is computed as weighted sum of backgrounds of all zones.
- The corrected probe balues can be calculated by subtracting the background

# Normalization is necessary because the raw intensities of labeled targets vary among arrays due to sources of experimental variability independent of level of expression