Distribution of 15 GHz to 86 GHz spectral indices (α_{15, 86}) for all sources in both the MOJAVE and our sample, and their corresponding quasar and BL Lac sub-samples. The 15 GHz total flux density was taken from integrated intensity of MOJAVE images. For each source, the 15 GHz observation taken at the closest date to our 86 GHz measurement was selected. Numbers in parentheses denote sample sizes.