# Length based double-sigmoid selectivity function

Source:`R/selectivity_funcs.R`

`double_sigmoid_length.Rd`

A hump-shaped selectivity function with a sigmoidal rise and an independent
sigmoidal drop-off. This drop-off is what distinguishes this from the
function `sigmoid_length()`

and it is intended to model the escape
of large individuals from the fishing gear.

## Arguments

- w
Vector of sizes.

- l25
the length which gives a selectivity of 25%.

- l50
the length which gives a selectivity of 50%.

- l50_right
the length which gives a selectivity of 50%.

- l25_right
the length which gives a selectivity of 25%.

- species_params
A list with the species params for the current species. Used to get at the length-weight parameters

`a`

and`b`

- ...
Unused

## Details

The selectivity is obtained as the product of two sigmoidal curves, one
rising and one dropping. The sigmoidal rise is based on the two parameters
`l25`

and `l50`

which determine the length at which 25% and 50%
of the stock is selected respectively. The sigmoidal drop-off is based on the
two parameters `l50_right`

and `l25_right`

which determine the
length at which the selectivity curve has dropped back to 50% and 25%
respectively.

As the size-based model is weight based, and this
selectivity function is length based, it uses the
length-weight parameters `a`

and `b`

to convert between length and weight.

## See also

Other selectivity functions:
`knife_edge()`

,
`sigmoid_length()`

,
`sigmoid_weight()`