For 1980 the percentage was determined by dividing the number of those who reported they took public transportation to work by the total number of persons answering this question. For 1990 the almost-but-not-quite analogous figure was determined by adding up those who said they took a "bus or trolley bus," the "subway or elevated," or a "railroad" to work and dividing it by the number of workers 16 and over. The figure used on the map is the percentage of change between the two proportions.