PowerWave Operator’s Manual 3-53
• Cubic curve fit: A curve fit which uses the equation “ax
3
+ bx
2
+ cx
+ d = y” to plot the standards values. This type of curve fit is
affected even less than the quadratic fit when any particular
standard has a poor value.
• 2-P (LOGIT/LOG): A curve fitted to the standard values, which is
characterized by a skewed sigmoidal (S-shaped) plot that eventually
becomes asymptotic to the upper and lower standard values. The
logistic equation is algebraically transformed to a simpler form in
which experimentally determined values are used for the responses
at concentrations of zero and infinity.
• Cubic Spline (C-Spline) curve fit: A piecewise polynomial
approximation consisting of joining a set of data points by a series of
straight lines, which is then smoothed by using a cubic fit.
• 4-Parameter Logistic or “4-P”: A curve fitted to the standard
values, which is characterized by a skewed sigmoidal (S-shaped)
plot that eventually becomes asymptotic to the upper and lower
standard values. The 4 parameters are: Left asymptote, Right
asymptote, Slope and Value at the Inflection point. This fit is most
recommended for immunoassay data, and is more exact than
Logit/Log.
Point to Point or “PT to PT”: A plot that connects each standard
point with a line, with no averaging of the values to “smooth” the
curve at each standard.
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