where a less serious failure mode receives a higher RPN than a more serious failure mode. The reason for this is that the rankings are ordinal scale numbers, and multiplication is not defined for ordinal numbers. The ordinal rankings only say that one ranking is better or worse than another, but not by how much. For instance, a ranking of "2" may not be twice as severe as a ranking of "1", or an "8" may not be twice as severe as a "4", but multiplication treats them as though they are. See Level of measurement for further discussion. Various solutions to this problems have been proposed, e.g., the use of fuzzy logic as an alternative to classic RPN model. In the new AIAG / VDA FMEA handbook (2019) the RPN approach was replaced by the AP (action priority). The FMEA worksheet is hard to produce, hard to understand and read, as well as hard to maintain. The use of neural network techniques to cluster and visualise failure modes were suggested starting from 2010. An alternative approach is to combine the traditional FMEA table with set of bow-tie diagrams. The diagrams provide a visualisation of the chains of cause and effect, while the FMEA table provides the detailed information about specific events. Types Functional: before design solutions are provided (or only on high level) functions can be evaluated on potential functional failure effects. General Mitigations ("design to" requirements) can be proposed to limit consequence of functional failures or limit the probability of occurrence in this early development. It is based on a functional breakdown of a system. This type may also be used for Software evaluation. Concept design / hardware: analysis of systems or subsystems in the early design concept stages to analyse the failure mechanisms and lower level functional failures, specially to different concept solutions in more detail. It may be used in trade-off studies. Detailed design / hardware: analysis of products prior to production. These are the most detailed (in MIL 1629 called Piece-Part or Hardware FMEA) FMEAs and used to identify any possible hardware (or other) failure mode up to the lowest part level. It should be based on hardware breakdown (e.g. the BoM = bill of materials). Any failure effect severity, failure prevention (mitigation), failure detection and diagnostics may be fully analyzed in this FMEA. Process: analysis of manufacturing and assembly processes. Both quality and reliability