**Syntax**

` >>>> PERTURB: (-) alpha (%)`

**Parent Command**

all third-level commands in block` > PARAMETER`

**Subcommand**

` -`

**Description**

This command specifies the perturbation factor *alpha* for numerical computation of the Jacobian matrix. The columns of the Jacobian matrix are calculated by perturbing the corresponding parameter p by a small amount dp, and taking either a forward or centered finite difference quotient. The perturbation is usually a fraction of the parameter value itself:

dp = *alpha* * pThe default value for *alpha* is 1 %, which can be changed globally for all parameters using the third-level command `>>> PERTURB,` or individually for each parameter using the fourth-level command `>>>> PERTURB.` A negative value can be provided to specify a constant perturbation, independent of parameter value:

dp = |-*alpha*|This option is sometimes required for parameters having either very small or very large values (such as initial pressure, generation times, or residual gas saturation).

**Example**

` > PARAMETER
>> TIME at which production rate changes
>>> SOURCE: WEL_3
>>>> INDEX of array F1 : 5
>>>> initial GUESS : 1.7E8 seconds
>>>> PERTURB by constantly : -3.6E3 seconds
<<<<
<<<
<<
> COMPUTATION
>> JACOBIAN
>>> all other parameters are PERTURBed by : 2 % of their values
<<<
<<`

**See Also**

` >>> PERTURB
`