The concept of layered security is the idea that attacks that are able to bypass one layer of security will eventually be caught by a subsequent layer. The effectiveness of this approach is typically calculated as the product of the individual layers failure rate. For example, layer 1 is assumed to miss 10% of the attacks and layer 2 are assumed to have a 10% success rate of missing 10%. The combined failure rate of these two layers is estimated to be 10% x 10% = 1%.”]
Source: https://informationsecuritybuzz.com/news/how-well-does-defense-in-depth-really-work/