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Comment by raverbashing | Hacker News Reader

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Scientists keep saying and repeating how correlation doesn't mean anything to the point it's become a joke

Now they are dabbling about a spurious correlation in a small section of the data? For real?

No, really, if anyone would be trying to prove anything based on that correlation alone they would have been laughed and dismissed by the majority of the scientific community

But suddenly when it's to reject a result then it's important?

> points out that Jackson’s team could have misused a common data-processing technique called the Fourier transform... The error, Harry writes, has to do with the technical assumption that the input data signal be “cyclical,”

Correct. Not to mention the sample is too short (and the even is short as well and there's noise and the sampling rate is not too big).

If you play with FFT on these conditions you start seeing things that "aren't there"

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Scientists don't say correlation doesn't mean anything. What they do say is that correlation doesn't imply causation.

If the basic model underlying the original LIGO finding would predict no correlation between the recordings, then a significant amount of correlation should lead scientists to revise the model, and reexamine the findings. Very much standard scientific practice.

There are several possible sources of correlation on the "impulse response" of the LIGO detectors (the whole detector, from building construction to the electronics and detectors in itself) so it is not surprising that might happen. Or maybe it's something in the source of the signal

But the signal is still stronger than the noise

Besides the "poking the data in weird ways until it stops meaning anything" issue

No. The word "Correlation" as used in the article to discuss correlations in the data generated by two remote detectors has a very specific technical meaning. It's not simply one of the words in the trope "correlation does not imply causation."

Correlation as used here and put simply means that when the signal coming out of one the detectors increases, the signal coming out of the other detector (which is physically separated from the first detector by a very large distance) also increases, and when the signal coming out of the first detector decreases the signal coming out of the second detector also decreases.

The existence of that mathematical correlation between those two signals coming out of those two remote detectors absolutely implies there is a causation to the correlation (yes, mathematical correlation can imply a causation, even while it alone is not able to indicate what that causation is).

The question at hand is not whether there is a correlation or whether there is a causation. The question is what is the causation for the correlation and whether that causation is a gravity wave or something else.

The odds are extremely good that that causation is a gravity wave, but it's a big enough experiment and a big enough result that it's worth looking very closely at the analyses that argue in favor of it being a gravity wave (and those that argue against it).

... what? This is precisely the correlation meant in the aphorism you mentioned and is also the only sense of the word correlation of which I am aware of. Signals can be similar (correlated) purely by chance and this cannot be taken to imply a causative link between them.

If a correlation is consistent, some sort of causation is quite likely. There are of cause spurious correlations but in physics experiments we can mostly get rid of them by repeating the experiment several times, unlike in economics for example.

The more important point, however, is that a correlation between A and B does not imply that A causes B or B causes A. It could also be that unknown factor C causes A and B.

Agreed, though repeated measurements weren't mentioned so I thought it was important to emphasize that finding a correlation in one measurement cannot be taken to imply causation, which sounded like what OP was saying.

Your point is a very good one, though. If we view causation through a causal graph, such that vertices are variables and direct causal links are arrows (directed edges), a statement "correlation doesn't imply causation" can be taken to mean several things:

  * From correlation, we cannot determine the direction of the arrow.
  * From correlation, we cannot claim there *is* a (direct) arrow between two vertices.
  * From correlation, we cannot claim there is even a path from one vertex to the other.

Indeed. I think it is also important to note that the unknown factor can be something very abstract. For example if you measure the oscillations of two pendulums with the same properties, they will also be correlated, but simply because they are driven by the same underlying physics...

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