Reading a Trend Line

Reading a trend line means establishing what is measured, over what period, against what baseline, before accepting the direction the line appears to show.

A line labeled "arrests" climbing steadily may record more crime, more police, a changed definition of the offense, or better record-keeping — the line cannot distinguish them, and the label will not say. Work through four checks. First, what is counted: arrests are not offenses, and reported offenses are not offenses. Second, the period: a line starting at an unusual low will climb whatever happens next, so ask why the series begins where it does. Third, whether it is a rate or a count, per Soc 0178, since a population growing across the same years produces a rising count with a flat rate. Fourth, whether the definition held: an offense reclassified midway makes a step in the data that looks like a change in the world. A trend that survives all four checks is worth reading. Most published trends have not been checked on any of them.