## Errors in dependent variable

Suppose that you have two rulers,
one graduated in inches and the other in cm. You use the latter
one to assess how many cm go in 1, 2, 3 .... 10 inch. Reading
errors are inevitable and show up in the graph because the
data are not on a straight line anymore, some points being
too high and others too low. It is likely that on average
the error is nil, *i.e*. that points that are 1 mm
too high balance points that are 1 mm too low. The straight
line fitted by eye still gives a fair estimate of the relationship
between cm and inch, and the slope is likely to be 2.54, or
come very close to this.