noise figure in simulation is not the same as calculated

[noramalia]

noise figure

I would like to know how spectre calculate noise figure. Is there any formula that i am able to use it? this is because my noise figure in simulation is not the same as the calculated and as in the post-layout. Can anyone help me on this matter.
Thank you

 

[Dmitron]

noise figure

Unoise=En*F^0.5
En = noise density
F^0.5 - sqrt (bandwith)