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