One major component of urban smog is ozone, a highly reactive gas that forms when emissions from mobile and industrial sources react chemically in the presence of sunlight. One concern is that prolonged ozone exposure could cause noncancerous lung diseases such as fibrosis and emphysema. The NTP's bioassay project presented a unique opportunity for a collaboration between the HEI and the NTP. The HEI funded eight studies to investigate whether rats exposed to 0, 0.12, 0.5, or 1.0 parts per million (ppm) ozone, for 6 hrs/day, 5 days/week, for 20 months, developed alterations in their respiratory systems that were characteristic of chronic respiratory diseases. Dr. Catalano and colleagues at the Harvard School of Public Health describes the procedures that were used to analyze, interpret, and correlate the data from all of these studies. The investigators used a statistical method called median polish analysis to evaluate jointly the multiple endpoints selected for each disease surrogate. The statisticians chose median polish analysis because each endpoint was not measured on all rats and this technique accommodates missing data points. Using the median polish technique, they first developed composite scores that summarized the selected endpoints for each individual animal. Then they used these scores to analyze ozone concentration–response trends, and differences between rats exposed to ozone and control rats.