The phrase "common baseline" refers to a shared starting point or agreed upon reference point that everyone involved can use as a foundation or standard for comparison or analysis. Full definition
It seems to me we almost have in Serial and in Making A Murderer and in other similar programs — even where there's disagreement — there's a common baseline of a moral order. (relevantmagazine.com)
But, he outlined three threats to American democracy — economic inequality, racial divisions and the retreat of different segments of society into «bubbles», where opinions are not based on «some common baseline of facts». (nigeriapoliticsonline.com)
Note that because the correction is not necessarily zero for the respective baselines, each each time series needs to be independently normalised to get a common baseline. (realclimate.org)