Algorithm doesn’t capture Transfer Entropy when the lag is strictly above 1

[HaskDev0]

Dear Contributors/community,

I noticed an unwanted behavior (as I would name it) when trying to apply the te_compute function for the following case:

Assume X is some random time series. Then define Y as being np.roll(X,-1), meaning that Y would actually contain information about X one step before, so the function te_compute(X,Y, embedding=1) computes transfer entropy and it’s pretty high which is expected.

Now, of we compute te_compute(X,Y,embedding=2) it still sees that there is a transfer of information from Y to X.

But, if I define Y to be np.roll(X,-2), then te_compute(X,Y,embedding=2) doesn’t see the information transfer which is reflected in the output value being close to 0 or even negative.

Does somebody know where the problem might be?

[Gilnore]

I was looking over the code and it seems they didn’t include any lags of Y. So histories of Y isn’t accounted for at all.

I was going to raise a issue for that, and ask if that mattered, it seems your question have answered mine.

[HaskDev0]

@Gilnore , could you show this part of the code? Because lag=1 I still think is accounted because it matches to what is expected.

Or do you already have any ideas of including more lags into the code?

[Gilnore]

@Gilnore , could you show this part of the code? Because lag=1 I still think is accounted because it matches to what is expected.

Or do you already have any ideas of including more lags into the code?

I looked at the helper.py, if you interpret the k in xky space as the lags of x. The x and y as what they are, then substitute the word lag whenever they say embedding, it becomes clear.

How ever, I am currently being stumbled by the need of using the conditional joint entropy H(X(t)|X(hist),Y(hist)), since they appear to have done the search in the space encompassing all points, while having a joint probability means you look at the points in the intersection. Since I don’t know how to construct a set that represents the elements occurring with probability P(X(t)|X(hist)), and there’s no escaping that because of the joint space thing, I don’t have any ideas on where to add the extra lags.

Edit: I found this paper that wrote transfer entropy as some sum of 4 different entropies that don’t need conditional probabilities, I believe that would clear things up.

https://www.researchgate.net/publication/45641250_Transfer_entropy-a_model-free_measure_of_effective_connectivity_for_the_neurosciences

I read up on information theory a bit and found this is just an application of Shannon entropy chain rule. That means we’ll need to search in a joint space of Xt, X, and Y.

[Gilnore]

@Gilnore , could you show this part of the code? Because lag=1 I still think is accounted because it matches to what is expected.

Or do you already have any ideas of including more lags into the code?

Try replacing the currently used y with y and its embeddings everywhere. You might need to adjust some dimensions, but it should buff out. I realized this after looking up what Taken's embedding is.