gwhite wrote:brp wrote:I understand that estimating variance from only 2 points doesn't work. I would appreciate advice on how to obtain a model-averaged estimate (and 95% CI) for pup survival (averaged across years: 07-08 and 08-09).

Is this possible?

Thanks!

I think you got the discussion pointed in the wrong direction by bringing up variance components. What I think you want to do is to model average the estimates for 07-08 ad 08-09. When you do this model averaging, make sure the also check the box to have the variance-covariance of the model averaged estimates dumped to a DBF file. Then, compute the mean of the 2 model-averaged values. The variance of this mean is the sum of the 2X2 variance covariance matrix dvided by 4 (=2^2). SE of the mean is sqrt of the variance. CI could be computed in the usual way, i.e., +/1 2SD. You could also convert the mean and variance to the logit scale and compute CI on the logit scale and then back-transform.

Gary

If Gary has turned the boat in the right direction, then I see the issue. You can get there from here by applying the Delta method (appendix 2) to a function for the mean (X1+X2)/2. If you take a first-order Delta approximation to this function, you get

[0.5,0.5] * VC * T[0.5,0.5]

which amounts to taking the sum of the VC matrix and dividing by 4 (as per Gary's note).

There is an example of the application of the Delta method to a function of random variables in Chapter 6 (sidebar beginning on p. 68).