Combining the two rates To get the actual rate of interest (sometimes referred to as the composite or earnings rate) we combine the fixed rate and the inflation rate, using the equation in the example below.

The combined rate will never be less than zero. However, the combined rate can be lower than the fixed rate. If the inflation rate is negative (because we have deflation, not inflation), it can offset some of the fixed rate. If the inflation rate is so negative that it would take away more than the fixed rate, we don't let that happen. We stop at zero.

Yeah as the sibling commented. It is written in the link from the link where they bring up the formula.