Dear Soha,

>For example in my models I am getting big negative DCM.F values (up to -1200),

The absolute value of F is unimportant. What matters is how different F’s are for different models.

>DCM. Ce values are above 0.1

This means the error SD is sqrt(0.1)=0.316. This can only be interpreted in relation to the size of the signal.

Inferences about model parameters are based on the posterior density – these take into account both signal and noise.

Similarly, inferences about models.

> In DCM10 version, BMS algorithm compare the models based on AIC or BIC criteria or it started to take free energy difference into account?

We recommend using the Free Energy. See our recent paper: Neuroimage. 2012 January 2; 59(1): 319–330.

doi:
10.1016/j.neuroimage.2011.07.039

This paper also looks at model selection as a function of SNR.

Best wishes,

Will.

**From:**
SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] **On Behalf Of
**soha saleh

**Sent:** 06 February 2012 17:19

**To:** [log in to unmask]

**Subject:** Re: [SPM] Signal to Noise Ratio in DCM

Hello :

I have few questions about free energy and error variance in the data.

How big the DCM.F value can be, the same for the error variance. For example in my models I am getting big negative DCM.F values (up to -1200), also DCM.Ce values are above 0.1. Does that mean the data is very noisy? Also is the difference in log evidence
should be 3 minimum or the difference in DCM.F values?

In DCM10 version, BMS algorithm compare the models based on AIC or BIC criteria or it started to take free energy difference into account?

Thanks, I appreciate your help, and any hints you can give me regarding model selection.

Best regards,

Soha

Date: Fri, 3 Feb 2012 20:26:40 +0000

From: [log in to unmask]

Subject: Re: [SPM] Signal to Noise Ratio in DCM

To: [log in to unmask]

Dear Mobin,

DCM can handle any amount of signal to noise. To get an idea of how much noise can be tolerated for any particular
question, one would do simulations with increasing levels of noise and simulate the model comparison of interest. The noise level that renders the difference in free energy (DCM.F) less than 3 is the level of noise at which your data are too noisy to provide
evidence for one model over another. I think Klaas wrote an SPM routine to generate simulated data, given a model specification (a DCM structure).

I hope this helps.

With very best wishes,

Karl

**From:**
Mobin Anandwala [mailto:[log in to unmask]]

**Sent:** 03 February 2012 19:54

**To:** Friston, Karl

**Cc:** Stephan, Klaas; Penny, William

**Subject:** Signal to Noise Ratio in DCM

Dr. Friston Dr. Stephan and Dr. Penny

I asked the SPM list about how much Signal to noise ratio

can DCM handle when we have cognitive emotional tasks and nobody

answered since then.

Thank you

Mobin