Measuring interference effects is an important tool to investigate the processing of various stimulus properties.
Unfortunately, some interference effects are invisible to the methods used in the literature. Usually, a difference index is calculated from the congruent and incongruent conditions, and this index is tested against zero. However, if the direction of the interference is heterogeneous in that sample, i.e., while most participants show interference, but the direction of the interference effects differs, then the mean value of this index is zero.
To overcome this limitation, we introduced the dual index calculation method, in which method instead of calculating a single difference index and testing it against zero, the correlation of the congruent and incongruent condition indexes is tested. This dual index method is appropriate to reveal heterogeneous interference effects.
We recommend that in the analyses of interference effects, the dual index should also be calculated to find heterogeneous index. Note that dual index should be used together with the classic unified index, and not instead of it, because homogeneous interference effects (i.e., interference effects where the direction of the effect is the same for most participants) could be identified with the unified index.
Find the detailed description of the method and an application of it to discover a parity-number interference in our paper:
Attila Krajcsi, Gábor Lengyel, Ákos Laczkó (2018). Interference between number magnitude and parity: Discrete representation in number processing. Experimental Psychology. 65(2), 71–83. https://doi.org/10.1027/1618-3169/a000394
See an explanation why parity-number interference is important here.