# Logistic regression fitting problem

I'm trying to make a GLM with binomial distribution to evaluate the effect of the diameter at breast height and the occupation of tree termites in species of tree.

I'm using the following model:

model <- glm(ocupation ~ dap * specie, data = termites, family = binomial(link = logit))


My response variable is the occupation of termites on the tree (data of absence and presence); the independent variable is the dap and the interaction with kind of species.

But when I run the model the following errors appear:

Warning message:
glm.fit: odds adjusted numerically 0 or 1 occurred

Error in seq.default(1, length(levels(eval(as.name(qvar)))) - 1, by = 1) :

Error in solve.default(t(X) %*% W %*% X) :
system is computationally singular: reciprocal condition number = 1.93217e-38
>

Warning messages:
1: not plotting observations with leverage one:
10, 24, 26, 36, 37, 40, 43, 57, 71, 72, 73, 116, 138
2: not plotting observations with leverage one:
10, 24, 26, 36, 37, 40, 43, 57, 71, 72, 73, 116, 138
3: In sqrt(crit * p * (1 - hh)/hh) : NaNs produced
4: In sqrt(crit * p * (1 - hh)/hh) : NaNs produced


The same thing happens when I rotate the contrasts in the analysis

Error in seq.default(1, length(levels(eval(as.name(qvar)))) - 1, by = 1) :


Can someone help me? I understand very little about R.

• Could you say some more about how many cases there are with 'occupation' for each species (or, if 'occupation' is more frequent, how many lack 'occupation')? This type of result could either be something fairly simple (like not enough data for this type of analysis) or something more subtle like the Hauck-Donner effect.
– EdM
Nov 4, 2017 at 20:34
• I believe @EdM probably has the correct diagnosis. Nov 5, 2017 at 3:53
• If dap is diameter, as @EdM thinks, shall we assume that the research questions are something like 'what is the effect on infestation probability of having a larger diameter?' (a no interaction model), and then 'what is the increase of probability, if any, from what we would expect based on diameter due to being a particularly tasty or untasty species?' (an interaction model)? Nov 5, 2017 at 17:04
• @EdM, Yes. The DAP mean diameter. I have 45 cases with occupation and 141 cases without occupation. I think my sample is good, but i'm able to cntinue the analysis. I was doing it that way as you said, seeing the model first without interaction and then if there is to do with the interaction. But the reviewer said that the correct thing would be to evaluate everything in the same model, with DAP interacting with the plant species. Nov 6, 2017 at 13:40
• How many different species? Do any species either have 0 occupation or 100% occupation?
– EdM
Nov 6, 2017 at 14:03

Most likely you have a problem of perfect separation, meaning there are combinations of dap and specie that have all ocupation = 1 or all ocupation = 0. In the first case, the value of $$\log \frac{p(\texttt{ocupation}=1)}{p(\texttt{ocupation}=0)},$$ (the quantity that the linear part of your model is attempting to fit) that makes the data most likely is positive infinity (and in the second case, negative infinity). Since this is silly, the model has objected.

This problem occurs because although you know that the coefficients cannot really be large enough to predict such huge values for the logit, a model fit with maximum likelihood does not.

So much for the statistics. Here are some substantive suggestions:

1. Can you get a more informative dependent variable? Are termites an all or nothing affair, or are some infestations larger than others? Thinking of this quantity as a magnitude rather than presence/absence often opens up other more stable model choices.
2. Perhaps one or other of specie and dap is actually the driving force. How does the model look if you remove the interaction between them? I would guess that it works better.
3. We don't know what dap actually is, but if it is some feature of the environment and the 'species packing' assumption is approximately true, then specie will have a preferred range of dap. And if that is true, an interaction will never work well in a binary data context because there will always be missing combinations of variables.
• +1 I infer, however, from the question that dap is tree "diameter at breast height," which would be a continuous covariate. In that case evaluating the interaction would be important.
– EdM
Nov 5, 2017 at 9:49
• Ah, that sounds plausible. So there's probably a colinearity problem in there as well, if it's the case that species grow to species-typical ranges of diameters. Nov 5, 2017 at 16:54

Just need to add a bit more to the answer by @conjugateprior than can fit into a comment easily.

The usual rule of thumb for logistic regression is that you need about 15 events per predictor variable that you are considering in the model. For this purpose, "events" are the least frequent of occupied/not occupied, so that's 45 events. Also, all except one of your species counts as a separate predictor, so in the interaction model with 28 different species and the continuous dap predictor you are trying to evaluate 28 individual predictors PLUS 27 interactions between dap and species, a total of 55 predictors. So you don't even have 1 event per variable. Regularization, as suggested by @conjugateprior in a comment, might help, but at this point it seems that you really don't have enough information to proceed this way. Think carefully about what you are trying to accomplish; might there be some way to combine species (e.g., into genus) in some way to cut your number of predictors down? (That combination of species into groups, however, would need to be done without reference to the occupancy data.)

• +1 Grouping in a biologically reasonable manner is perhaps the best simple idea - essentially it's parameter tying for the ungrouped data, so one could also loosen the coupling by switching to random effects. Nov 6, 2017 at 15:53
• @conjugateprior I like your idea of treating the species as random effects, if that makes sense in terms of the hypothesis being examined; sounds like the interaction of dap with species would still have to be included, but that interaction would also then be a random effect.
– EdM
Nov 6, 2017 at 16:26
• Yes, @EdM, I can try to sort species together by gender or family, that's a good idea. But in case I continue to evaluate everything together in the same model or would be a model for each set of DAP per family? Nov 6, 2017 at 17:06
• @JakelyneSuélenBezerra, the problem is that each model needs to have about 15 events per predictor, so your 45 total events limit you to only 3 predictors (including interactions). If your primary interest is in the effect of dap on occupancy, you might succeed with a single mixed model that allows for random effects of species (or probably better, groups of species, as species with 0 events might lead to problems) and an interaction between dap and the (random-effects) groups. Using random, not fixed, effects would greatly cut down on the effective number of predictors in the model.
– EdM
Nov 6, 2017 at 18:18
• @EdM, how can I assemble a mixed model with each DAP data and plant group? I still do not know how to make mixed models. Nov 6, 2017 at 19:03