There were imperfections in our data collection. In some rooms occupants may have had the window open during the night measurements in winter. Perhaps the incidence of common cold was influenced by an influenza epidemic. These sources of error would shift our findings towards the null hypothesis, that there was no association between common cold infections and dorm crowdedness or ventilation rate. Our findings are robust in spite of these possible problems. Thus, it is likely that more measurements and more accurate data on types of airways infections would show an even stronger association.