Does Nonzero Correlation Between Pairs Imply Correlation Between All?

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Discussion Overview

The discussion revolves around the implications of nonzero correlations among pairs of scalar random variables. Participants explore whether the correlation between two variables implies correlation with a third variable, particularly in the context of zero-mean variables with nonzero variances. The scope includes theoretical reasoning and counterexamples.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if nonzero correlations between pairs of variables (a and b, b and c) imply a nonzero correlation between a and c.
  • Another participant provides a counterexample where a and c are uncorrelated, despite both being correlated with b, suggesting that the initial assumption does not hold.
  • A later post introduces a related problem involving uncorrelated variables a and b, asking if a third variable c could be found to create a correlation between a and b, indicating a search for a mathematical approach.
  • Participants express a desire for further insights and mathematical reasoning regarding the relationships among the variables.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There is disagreement regarding the implications of correlation among pairs of variables, with at least one counterexample provided that challenges the initial assumption.

Contextual Notes

The discussion highlights limitations in assumptions about correlation, particularly in the construction of counterexamples. The mathematical relationships and dependencies among the variables remain unresolved.

Who May Find This Useful

Readers interested in statistical relationships, correlation theory, and mathematical modeling of random variables may find this discussion relevant.

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If I have three scalar random variables: a, b and c, which are each zero-mean and have some nonzero variances, and I know:

1) The correlation between a and b is nonzero.

2) The correlation between b and c is nonzero.

Does this imply that the correlation between a and c is nonzero?

I feel like the answer must be yes, but I don't have any sound mathematical reasoning for it. Any advice would be greatly appreciated!
 
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Simple counterexample: Suppose that a and c are uncorrelated random variables each with zero mean and nonzero variance and suppose that bac, with α and γ non-zero constants. By construction, b is correlated with each of a and c, but a and c are (by construction) uncorrelated.
 
D H said:
Simple counterexample: Suppose that a and c are uncorrelated random variables each with zero mean and nonzero variance and suppose that bac, with α and γ non-zero constants. By construction, b is correlated with each of a and c, but a and c are (by construction) uncorrelated.

Many thanks for clearing that up so elegantly. It's easy when you know how!
 
Perhaps I can develop my understanding of a similar problem here without starting a new topic:

If, again, I have three scalar random variables a, b and c which are each zero-mean and have some nonzero variances... and in this case a and b are uncorrelated:

\mathcal{E} \left\{ ab^*\right\} = 0

where \mathcal{E}\left\{\right\} denotes expectation and * denotes complex conjugate (although the variables probably need not be complex for this example).

What I'd like to know is whether, in general, we can find a c which can sort of 'recorrelate' a and b:

\mathcal{E}\left\{ cab^*\right\} > 0

I can't seem to find such a case using numerical examples in Matlab, but I'd really like to figure out a proper mathematical approach to this. Any advice or insights would be very much appreciated!
 

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