People have a tendency to take the most recent, or the most easily available information and project the 'tend' into the future in a linear fashion - extrapolation.
Extrapolation is a dangerous game. Consider the following examples:
US Home Price Index: An extrapolation gone awry.
The volume of subprime loan originations had a 'straight line fit' from 2003 to 2006, but the line did not continue in 2007.
Exponential not extrapolation