### How to interpret fold-changes when the sample purity is not 100%

If your sample purity is less than 100%, it is necessary to take that into account when interpreting the fold-change values. Given a sample purity of %, and an observed fold-change of , following formula gives the actual fold-change that would be seen if the sample were 100% pure:

 fold-change in 100% pure sample (28.19)

For example, if the sample purity is 40%, and you have observed a fold-change of 3, then the fold-change in the 100% pure sample would have been:

 fold-change in 100% pure sample (28.20)

Figures 28.27 shows the 'true' fold changes for different observed fold-changes at different sample purities. Figure 28.28 zooms in for low-level amplifications and deletions.

Figure 28.27: The true fold-change in the 100% pure sample, for different observed fold-changes, as a function of sample purity.

Figure 28.28: Low-level amplifications and deletions: the true fold-change in the 100% pure sample, for different observed fold-changes, as a function of sample purity.