Semi-quantitative risk matrix approach usually uses a 4 by 4 matrix with broad quantitative bins for the frequency axis, while keeping the purely qualitative descriptions for the consequence axis. Three categories of risk are defined: Acceptable, Marginal, and Unacceptable. Even though the frequency axis is quantitative, the resulting risk for each region in the matrix is a mixed bag product of a numerical range and qualitative description, such as 10-4to 10-6)*Low. Again, there are no direct comparisons possible between the risk of some regions, such as 10-4 to 10-6)*Low and (< 106)*Moderate for example.

Reference: Thomas J. Altenbach, A Comparison of Risk Assessment Techniques from Qualitative to Quantitative, ASME Pressure and Piping Conference, 23-27, 1995