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Question about the secret weapon

Micah Wright writes:

I first encountered your explanation of secret weapon plots while I was browsing your blog in grad school, and later in your 2007 book with Jennifer Hill. I found them immediately compelling and intuitive, but I have been met with a lot of confusion and some skepticism when I’ve tried to use them. I’m uncertain as to whether it’s me that’s confused, or whether my audience doesn’t get it. I should note that my formal statistical training is somewhat limited—while I was able to take a couple of stats courses during my masters, I’ve had to learn quite a bit on the side, which makes me skeptical as to whether or not I actually understand what I’m doing.

My main question is this: when using the secret weapon, does it make sense to subset the data across any arbitrary variable of interest, as long as you want to see if the effects of other variables vary across its range? My specific case concerns tree growth (ring widths). I’m interested to see how the effect of competition (crowding and other indices) on growth varies at different temperatures, and if these patterns change in different locations (there are two locations). To do this, I subset the growth data in two steps: first by location, then by each degree of temperature, which I rounded to the nearest integer. I then ran the same linear model on each subset. The model had growth as the response, and competition variables as predictors, which were standardized. I’ve attached the resulting figure [see above], which plots the change in effect for each predictor over the range of temperature.

My reply: I like these graphs! In future you might try a 6 x K grid, where K is the number of different things you’re plotting. That is, right now you’re wasting one of your directions because your 2 x 3 grid doesn’t mean anything. These plots are fine, but if you have more information for each of these predictors, you can consider plotting the existing information as six little graphs stacked vertically and then you’ll have room for additional columns. In addition, you should make the tick marks much smaller, put the labels closer to the axes, and reduce the number of axis labels, especially on the vertical axes. For example, (0.0, 0.3, 0.6, 0.9) can be replaced by labels at 0, 0.5, 1.

Regarding the larger issue of, what is the secret weapon, as always I see it as an approximation to a full model that bridges the different analyses. It’s a sort of nonparametric analysis. You should be able to get better estimates by using some modeling, but a lot of that smoothing can be done visually anyway, so the secret weapon gets you most of the way there, and in my view it’s much much better than the usual alternative of fitting a single model to all the data without letting all the coefficients vary.

2 Comments

  1. Keith O'Rourke says:

    Micah: If you really want to confuse folks take this to the extreme of individual observations and or display log (marginal) likelihoods for the parameters rather than intervals. http://andrewgelman.com/wp-content/uploads/2011/05/plot13.pdf

    The math is _simply_ for independent observations the likelihood factorizes (L1 * L2 * … Ln) so on the log scale the individual log likelihoods add to the total log likelihood that adds to the log prior to get the log posterior. With more than two parameters visualizing this (getting the secret weapon) will be too hard so marginalize out all but one parameter and plot curves for that one parameter. Then in turn other parameters. You could get intervals for that parameter from these but unless log likelihoods are approximately quadratic they may be misleading.

    But what to make of such plots seems hard for many to _see_.

    If you do the above not for individual observations for groups of observations meeting some criterion say temperature and location – you get what you did above.

  2. Andrew says:

    I’m curious what software you use/recommend to make these?

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