The PERT distribution assigns very small probability to extreme values, particularly to the extreme furthest away from the most likely value if the distribution is strongly skewed.
The Modified PERT distribution  was proposed to provide more control on how much probability is assigned to tail values of the distribution.
The modified-PERT introduces a fourth parameter  that controls the weight of the most likely value in the determination of the mean:
Typically, values of between 2 and 3.5 are used for  and have the effect of flattening the density curve.
This is useful for highly skewed distributions where the distances  and  are of very different sizes.

The modified-PERT distribution has been implemented in several simulation packages:
ModelRisk – risk analysis add-in for Excel.

Primavera risk analysis – project risk analysis simulation tool.

R (programming language) - open-source open source programming language for statistical computing.

Tamara  – project risk analysis simulation tool.

Wolfram Mathematica – mathematical symbolic computation program.
The PERT distribution assigns very small probability to extreme values, particularly to the extreme furthest away from the most likely value if the distribution is strongly skewed. The Modified PERT distribution  was proposed to provide more control on how much probability is assigned to tail values of the distribution. The modified-PERT introduces a fourth parameter  that controls the weight of the most likely value in the determination of the mean:
Typically, values of between 2 and 3.5 are used for  and have the effect of flattening the density curve. This is useful for highly skewed distributions where the distances  and  are of very different sizes.
The modified-PERT distribution has been implemented in several simulation packages:
ModelRisk – risk analysis add-in for Excel.
Primavera risk analysis – project risk analysis simulation tool.
R (programming language) - open-source open source programming language for statistical computing.
Tamara  – project risk analysis simulation tool.
Wolfram Mathematica – mathematical symbolic computation program.
PERT distribution