Correlation matrix - Cancer types

http://www.livestockgenomics.csiro.au/Genome_to_Phenome/  - click on Figure 8

 

This correlation matrix is examining possible correlations between types of cancers.  In a typical correlation matrix, there is a diagonal line of correlation=1 (in this case red, meaning that brain cancer correlates with brain cancer, etc.)  What we are looking for are other red boxes underneath and above the diagonal line.  It is interesting and perplexing to see that “Brain_Normal” is seemlingly correlated with “Brain_Cancer”, “Breast_Cancer” and “Colon_Cancer”.  However, since this diagonal correlation line is beneath the standard red correlation line, it means these variables are negatively correlated – so, if you have a “normal” brain, you are likely not to have brain cancer, breast cancer, or colon cancer.  Again, it is important to note that this matrix depicts correlation, not necessarily causation.