Question: Could anyone give me hints as to a model framework that can be used in the following setting:

The outcome A is dichotomous. I want to investigate the effect of a continuous variable B and a continous variable C on A in a longitudinal setting. So far so good. 

The problem is that C depends on B, in that values of B above a threshold (Unfortunately we do not know the threshold and the threshold itself might not be constant over time) will change C. The change in C will furthermore reduce B in the next measurement.

How can I decipher the isolated effect og B and C on A? 

Would a mixed-logistic-regression model be OK?

ie;t=time

A~B\*t+C\*t+B\*C\*t

Any other ideas?

//M

Answer: 
You have clearly stated a part of your model:

> 
> C depends on B, in that values of B above a threshold will change C. The change in C will furthermore reduce B in the next measurement.
> 
> 
> 

By "next measurement" I understand you mean next *in time.* Let's index time as $t = 0, 1, 2, \ldots$. Then the dependence of C on B sounds like a contemporaneous one. If we adopt a simple linear model (which can readily be expanded to incorporate covariates and variable but predetermined thresholds) and let $u$ be a constant threshold,

$$C(t) = \beta\_1 I\_{B(t) \gt u} + \epsilon$$

with random deviations $\epsilon$ (which I won't bother to index; you know the drill). Here, $I$ is the indicator function.

Also,

$$B(t+1) = B(t) - \beta\_2 (C(t) - C(t-1)) + \delta$$

and again $\delta$ represents random (independent) deviations. I'm stuck here because you haven't specified more precisely just *how* B changes in response to a change in C; I have merely provided one possible interpretation.

Finally,

$$A(t) = \beta\_3 B(t) + \beta\_4 C(t) + \beta\_5 + \gamma$$

with independent random deviations $\gamma$.

The presence of that indicator function in the first formula is problematic: it makes this a nonlinear problem. However, this seems to be an essential feature of the situation; I would be loth to ignore it in the name of simplicity or ease of calculation (although both are important considerations). The lags $B(t+1) - B(t)$ and $C(t) - C(t-1)$ also point towards autoregressive models, another complication. Because of these issues, the most tractable approach might be with Bayesian techniques: parameterize the distributions of $\epsilon$, $\delta$, and $\gamma$, provide priors for those parameters and the $\beta$s, and let the machinery (*e.g.*, WinBUGS or RBUGS) roll.
Your problem is one of multi-collinear regressors (since B and C are correlated). I would suggest that you look at the answers to the question: [Dealing with correlated regressors](https://stats.stackexchange.com/q/3561/28). 

The following paper may also be relevant in your context: [Using principal components for estimating logistic regression with high-dimensional multicollinear data](http://dx.doi.org/10.1016/j.csda.2005.03.011)