Global Indices (MD, PSD, LV..)

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Indices are statistical calculations on the basis of all measured values ("global")
Note: Values may differ slightly from the values in the device program. The reason for this is different rounding methods. Some devices (e.g., HFA) also use different formulas or weight the points differently. This results in greater differences. PeriData weights each measured value equally. This makes it easier to compare findings from different devices.

N: Count of all evaluated measured points. Because the region of the blind spot is eliminated from the calculations, the number may be smaller than the point count in the pattern

MD: Mean Deviation (mean defect) = mean value of all total deviation (defect depth) values. The lower the MD, the more the visual field is damaged.  MD+: option "Defect value sign (+)" active - Note the comment above concerning differences to device values

PSD: Pattern Standard Deviation = standard deviation of total deviation values (defect depth). The higher the index, the greater are the differences between the total deviation (defect depth) of different points

CPSD: Corrected Pattern Standard Deviation = index PSD, corrected by the accidental measurement error SF. Missing if SF was not measured.

FD: Functional Defect = mean value of converted total deviation values (defect depth). Instead of dB = decibel a non-logarithmic percentage value (functional equivalent) is used. Normal value 0%, maximum loss -100%.
A loss of -6 dB is equivalent to a functional reduction of about -50%, a loss of -12 dB equals about -75%. The conversion is based on an empirical study (Bartz-Schmidt KU, Weber J: Comparison of spatial thresholds and intensity thresholds in glaucoma, Int Ophthalmol 1993, 17:171-178). Note: Because the averaging is done after the conversion, the relation of MD and FD differs between examinations

SF: Short-term Fluctuation = variation value at points with double measurements. This value is the measurement error in perimetry. Missing, if less than 10 double measurements were made.

Octopus style indices: If this option is active, the squared values LV/CLV (Loss Variance) will be shown instead of PSD/CPSD


CONCLUSION: Indices are providing information about the height and the spread of total deviation values (defect depth). The index MD can show the temporal course in the trend image "box plot"   -- For a single visual field, similar information can be taken from the Bebié curve (cumulative defect curve). Indices contain no topographical information. For a complete evaluation the Color Defect Map is needed in addition.