Frequency of recognised χ² double minima for simulated IRAS and Planck observations. Each frame corresponds to one combination of the temperature and the spectral index that were inputs of the simulation. The horizontal axis gives the 350 μm intensity (before noise is added) and the vertical axis is the factor K₁₀₀ (smaller numbers correspond to a larger weight of the 100 μm point). The colour scale gives the frequency of the double χ² minima as a percentage of all noise realisations with the corresponding values of S(350   μm) and K₁₀₀.