The I Chart plots the period's individual values. If there is more than one value in a period grouping then the plotted point is the sum of the values in the period. This particular I chart calculates the number of sigmas to use based on the number of points that are plotted.
Counts such as Newborn Discharges or Census are excellent examples of this type of data. We would like to see how the number of Newborn Discharges has been varying over time.
It is important to realize that the plotted value is the sum of the values in the period. If you have several classes in your data that are reporting counts this is likely the chart you are interested in. However if your data has the rates for those classes you likely would not like to see the sum of those rates as a plotted point. The Multiple Values/period charts will take an average or mean of these values.
Other charts that work with single values per period are Bar Chart, C Chart, C Chart 3-Sigma, C Chart 2-Sigma, I Chart, I Chart 3-Sigma, I Chart 2-Sigma, ImR Chart, ImR Chart 3-Sigma, ImR Chart 2-Sigma, Trend Chart and Run Chart.
The sigma value used is determined by the T Table.