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During the "sizing" step each data point (time position in a trace file or pixel position in an image) obtains a base size using the known base sizes from the size standard channel or lane.

Local Southern Algorithm (Southern, 1979)
The Local Southern algorithm allows for exact peak sizing of any fragment with a length between the shortest and the longest size standard fragment. Peaks occurring above the highest and below the smallest size standard peak cannot be exactly sized. They are sized by linear regression using the two closest standard peaks.

For determining the size of a fragment the Local Southern algorithm uses two size standard peaks above and two size standard peaks below the unknown sample fragment.

f(m) = c / (m - M0) + L0

A curve is calculated from two standard peaks below and one above the mobility of the unknown fragment. The constants M0, L0 and c are calculated using these three peaks.

The same procedure is used to calculate a curve from one standard peak below and two standard peaks above the mobility of the unsized peak.

Then the mean value for both curves is calculated and thus the size of the unsized sample peak is determined.