[Gate-users] Calculating standard error of the mean from multiple files

[florian.bretin at ulg.ac.be]

Dear GATE experts,

I am simulating dose maps from multiple CT acquisitions and I am trying to calculate the final standard error of the mean of the summed dose maps. 

The GATE manual states regarding this the following: 

"The dose/edep squared is used to calculate the uncertainty when the output
from several files are added. The uncertainty is the relative statistical uncertainty. "SquaredDose" flag allows to store the sum of
squared dose (or energy). It is very useful when using GATE on several workstations with numerous jobs. To compute the final
uncertainty, you only have to sum the dose map and the squared dose map to estimate the final uncertainty according to the
uncertainty equations."

The equations for the calculation of the uncertainty are given on page 98 of the GATE manual (GATE_v6.2_Complete_Users_Guide.pdf).

Am I correct to assume that N equals the number of hits (I did set the flag to collect number of hits!)? My sample variance is already negative, since the squared dose values are far lower than values stored in the dosemap (all values very small, since only a fraction of real photons are simulated!). This leads to complex numbers when I try to calculate the standard mean of the error! 

It would be much appreciated if somebody could point me in the right direction or share his experience! 

Cheers,
Florian