# Tips for Using the Calculator

The interactive calculator will provide an estimate of the number of invasive group A streptococcal (GAS) disease cases that might be expected in a given month in a county or region with characteristics that you specify. Enter the month of interest, the number of invasive GAS infections in that month and the previous month, the average temperature of your state during the previous month, characteristics of your county/region, including population and area in square miles, and factors related to the risk of invasive GAS infection, such as the prevalence of HIV-infection and injection drug use among GAS cases in your area. The population of the area must be 250,000 persons or larger; smaller counties may be combined. Some answers to common questions about inputting data are given in the Frequently Asked Questions (FAQ) page.

When you have entered the data required, press “Get Results.” The next page shows the model inputs and the results. The expected number of cases is the average number of invasive GAS cases for a one month period in a region with similar characteristics. The calculator shows you the difference between the observed and expected number of cases. The p-value is the probability of finding a number of cases greater than or equal to that observed. Smaller p-values indicate unlikely occurrences. For example, a p-value of 0.01 indicates that the observed number of cases would be expected to occur only 1 time out of 100 months, or once every 8 years.

We suggest that you run several model options with varying data to give a range of predictions. For example, you may enter high and low estimates for a particular variable and compare results. The results page may be printed or exported to a Microsoft Excel spreadsheet. You may use the “Back” button on your browser to re-enter data.

### How the calculator was developed

The calculator is based on a regression model of data from CDC’s Active Bacterial Core surveillance (ABCs), an active, population-based surveillance system for invasive group A streptococcal infections. The model used data collected from 1989 through 2002 from diverse populations in 9 states (CA, CO, CT, GA, MD, MN, NY, OR, TN), including over 50 regions ranging in size from 250,000 to 1.3 million persons. The calculator estimates the average number of invasive GAS cases expected during a one month period in a region with given population characteristics.