Of the nation’s 1,183 cities and towns, places with populations of over 30,000 and where cost for radio advertisements was below $111 per point were shortlisted. The 151 shortlisted municipalities were matched into pairs based on criteria thought to affect election competitiveness. These matching criteria were voter turnout and incumbent vote share in the previous mayoral election, whether mayoral elections are partisan or nonpartisan, and whether the 2005 mayoral election was contested. This resulted in 28 pairs of cities with one of each pair randomly assigned to control and the other to treatment. From these, 33 municipalities (16 treatment and 17 control) where an incumbent mayor was running opposed were selected as sample cities. The radio advertisement was played in treatment cities only.
In November 2006, 105 municipalities held mayoral elections. Using the process as in 2005, 11 pairs of municipalities were created, half of which were randomly assigned to the treatment group. Of these, 16 cities—seven in the treatment group and nine in the control group—featured elections in which incumbent mayors ran opposed in 2006.