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Symbols (squares): Measured front speed from archaeological data. Lines: Upper and lower error bands (dotted and dashed lines respectively) predicted when considering the 
C.L. intervals for 
and 
for (a) the time-delayed model and (b) the non-delayed model. The solid line in (a) and the dash-dotted line in (b) correspond to the mean prediction, i.e., they are the same as in Fig. 2b. (
, 
and 
).
