The Richards model for incremental data
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The Richards curve or generalized logistic is a widely used growth model that will fit a wide range of S-shaped growth curves. There are both 4 and 5 parameter versions in common use. The logistic curve is symmetrical about the point of inflection of the curve. To deal with situations where the growth curve is asymmetrical, Richards (1959) added an additional parameter.
There are a number of different ways that this equation can be written (see Logistic curve). When using incremental data the version used is
where t is age,
where k is the growth rate,
b a scaling parameter,
δ, a parameter that in part determines the point of inflection on the y axis and
L∞, termed 'L infinity', is the asymptotic length at which growth is zero.
For incremental data Growth II will not estimate d, which must be supplied prior to fitting.