|Figure 1: Fictitious data showing spurious correlation between height and reading ability|
|Figure 2: Model showing how a spurious correlation between height and reading arises because both are affected by age|
|Figure 3: 95% confidence interval around estimated correlation of .5, with different sample sizes|
|Figure 4: Correlations obtained in samples of dyslexic (red) and controls (blue) for 20 runs of simulation with N = 30 per group.|
|Figure 5: Models of causal relationships underlying observed correlation between auditory deficit and language impairment|
- We found that, when studying heritable conditions, it is useful to include data on parents or siblings. Models differ in predictions about how measures of genetic risk - for instance, family history, or presence of specific genetic variants - relate to A (auditory deficit) and B (language impairment) in the child. This approach is illustrated in this paper. Interestingly, we found that the causal model that is often implicitly assumed, which we termed the Endophenotype model, did not fit the data, but nor did the spurious correlation model, which corresponds here to the Pleiotropy model.
- There may be other groups that can be informative: for instance, if you think auditory deficits are key in causing language problems, it may be worth including children with hearing loss in a study - see this paper for an example of this approach using converging evidence.
- Longitudinal data can help distinguish whether A causes B or B causes A.
- Training studies are particularly powerful, in allowing one to manipulate A and see if it changes B.
P.S. 9th July 2012
I've written a little tutorial on simulating data using R to illustrate some of these points. No prior knowledge of R required. see: http://tinyurl.com/d2868cg
If you liked this post, you may also be interested in my other posts on statistical topics:
Getting genetic effect sizes in perspective
The joys of inventing data
A short nerdy post about the use of percentiles
The difference between p < .05 and a screening test