Why are small numbers of cases or events a concern when looking at health data?
Questions about diseases and conditions such as asthma often lead to looking at the disease in small geographic areas such as neighborhoods and ZIP Codes. When the focus is on small areas, the number of health events is likely to be small. When dealing with small numbers of health events, privacy and confidentiality need to be maintained. In addition, people need to be aware that seemingly small changes in the number of events can dramatically change calculated rates. No information should be presented in these tables that may identify a person or release confidential information about that person. In the tables of asthma hospital discharges by ZIP Code, the number of hospital discharges is not shown if the population is 33 or less (i.e., equal to or less than an average of 33 per year or 100 over a three-year period), or if the number of hospital discharges is one or two. A count of no hospital discharges is shown in the tables because a count of zero is not a threat to confidentialit
