Users are often frustrated when a touch screen monitor inaccurately records their touches. Enlarging touch sensitive regions improves touch accuracy, but few specific guidelines are available in the literature. A controlled field experiment determined the effect of target location and visual target size on user accuracy and empirically derived quantitative guidelines for determining touch target size based. on target location. The experiment was conducted in a grocery store using a piezo-electric monitor in a public access kiosk. Participants pressed the screen as target squares appeared one at a time. Visual target size, horizontal viewing location, and screen sector of target were varied. X and Y offset between the target center and the touch location were recorded. Results showed Significant differences caused by target sector in X offsets among columns and in Y offsets among rows, but no differences caused by target size. Results showed that persons tended to touch below the target, with touch distance increasing as the location of the target moved down the screen. To a lesser extent, persons tended to touch toward the sides of the screen. Using collected data for each of nine screen sectors, graphs were prepared showing the relationship between touch target size and expected accuracy under harsh conditions. These empirically derived, quantitative guidelines will help designers plan for the worst case and create screens that decrease user errors and frustration. / Master of Science