The calculation examines the developement of the visual field over time. The calculation is performed separately for each particular point in the visual field and for each visual field index. The mathematical method is linear regression analysis and is a statistical standard.
Basis is an (invisible) plot of the values versus time. A line is calculated that is closest to all points. This is the regression line. The line is the most probable course of the value, because measurement error and other fluctuations are smoothed out.
The start point and end point of the regression line are the "statistical start value"and the "statistical end value" and are different from the real measured values. The difference between these two values is the change value. Some image types use the change value, other image types with double representation (GATT, NATT) use the start value and the end value.
The slope of the regression line can be given in units per year (mostly dB/year). The probability that the line is not accidental is called signifikance (of change) and also used in the trend images.
With 2 visual fields, trend images are possible, but without a significance statement which needs at least 3 values