# Forward and backward stepwise regression (AIC) for negative binomial regression (with real data)

I am doing some count data analysis. The data is in this link.

Column A is the count data, and other columns are the independent variables. At first I used Poisson regression to analyze it:

m0<-glm(A~.,data=d,family="poisson")
summary(m0)


We see that the residual deviance is greater than the degrees of freedom so that we have over-dispersion:

Call:
glm(formula = A ~ ., family = "poisson", data = d)

Deviance Residuals:
Min        1Q    Median        3Q       Max
-28.8979   -4.5110    0.0384    5.4327   20.3809

Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept)  8.7054842  0.9100882   9.566  < 2e-16 ***
B           -0.1173783  0.0172330  -6.811 9.68e-12 ***
C            0.0864118  0.0182549   4.734 2.21e-06 ***
D            0.1169891  0.0301960   3.874 0.000107 ***
E            0.0738377  0.0098131   7.524 5.30e-14 ***
F            0.3814588  0.0093793  40.670  < 2e-16 ***
G           -0.3712263  0.0274347 -13.531  < 2e-16 ***
H           -0.0694672  0.0022137 -31.380  < 2e-16 ***
I           -0.0634488  0.0034316 -18.490  < 2e-16 ***
J           -0.0098852  0.0064538  -1.532 0.125602
K           -0.1105270  0.0128016  -8.634  < 2e-16 ***
L           -0.3304606  0.0155454 -21.258  < 2e-16 ***
M            0.2274175  0.0259872   8.751  < 2e-16 ***
N            0.2922063  0.0174406  16.754  < 2e-16 ***
O            0.1179708  0.0119332   9.886  < 2e-16 ***
P            0.0618776  0.0260646   2.374 0.017596 *
Q           -0.0303909  0.0060060  -5.060 4.19e-07 ***
R           -0.0018939  0.0037642  -0.503 0.614864
S            0.0383040  0.0065841   5.818 5.97e-09 ***
T            0.0318111  0.0116611   2.728 0.006373 **
U            0.2421129  0.0145502  16.640  < 2e-16 ***
V            0.1782144  0.0090858  19.615  < 2e-16 ***
W           -0.5105135  0.0258136 -19.777  < 2e-16 ***
X           -0.0583590  0.0043641 -13.373  < 2e-16 ***
Y           -0.1554609  0.0042604 -36.489  < 2e-16 ***
Z            0.0064478  0.0001184  54.459  < 2e-16 ***
AA           0.3880479  0.0164929  23.528  < 2e-16 ***
AB           0.1511362  0.0050471  29.945  < 2e-16 ***
AC           0.0557880  0.0181129   3.080 0.002070 **
AD          -0.6569099  0.0368771 -17.813  < 2e-16 ***
AE          -0.0040679  0.0003960 -10.273  < 2e-16 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

(Dispersion parameter for poisson family taken to be 1)

Null deviance: 97109.0  on 56  degrees of freedom
Residual deviance:  5649.7  on 26  degrees of freedom
AIC: 6117.1

Number of Fisher Scoring iterations: 6


Then I think I should use negative binomial regression for the over-dispersion data. Since you can see I have many independent variables, and I wanted to select the important variables. And I decide to use stepwise regression to select the independent variable. At first, I create a full model:

full.model <- glm.nb(A~., data=d,maxit=1000)
# when not indicating maxit, or maxit=100, it shows Warning messages: 1: glm.fit: algorithm did not converge; 2: In glm.nb(A ~ ., data = d, maxit = 100) : alternation limit reached


When indicating maxit=1000, the warning message disappears.

summary(full.model)

Call:
glm.nb(formula = A ~ ., data = d, maxit = 1000, init.theta = 2.730327193,

Deviance Residuals:
Min       1Q   Median       3Q      Max
-2.5816  -0.8893  -0.3177   0.4882   1.9073

Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) 11.8228596  8.3004322   1.424  0.15434
B           -0.2592324  0.1732782  -1.496  0.13464
C            0.2890696  0.1928685   1.499  0.13393
D            0.3136262  0.3331182   0.941  0.34646
E            0.3764257  0.1313142   2.867  0.00415 **
F            0.3257785  0.1448082   2.250  0.02447 *
G           -0.7585881  0.2343529  -3.237  0.00121 **
H           -0.0714660  0.0343683  -2.079  0.03758 *
I           -0.1050681  0.0357237  -2.941  0.00327 **
J            0.0810292  0.0566905   1.429  0.15291
K            0.2582978  0.1574582   1.640  0.10092
L           -0.2009784  0.1543773  -1.302  0.19296
M           -0.2359658  0.3216941  -0.734  0.46325
N           -0.0689036  0.1910518  -0.361  0.71836
O            0.0514983  0.1383610   0.372  0.70974
P            0.1843138  0.3253483   0.567  0.57105
Q            0.0198326  0.0509651   0.389  0.69717
R            0.0892239  0.0459729   1.941  0.05228 .
S           -0.0430981  0.0856391  -0.503  0.61479
T            0.2205653  0.1408009   1.567  0.11723
U            0.2450243  0.1838056   1.333  0.18251
V            0.1253683  0.0888411   1.411  0.15820
W           -0.4636739  0.2348172  -1.975  0.04831 *
X           -0.0623290  0.0508299  -1.226  0.22011
Y           -0.0939878  0.0606831  -1.549  0.12142
Z            0.0019530  0.0015143   1.290  0.19716
AA          -0.2888123  0.2449085  -1.179  0.23829
AB           0.1185890  0.0696343   1.703  0.08856 .
AC          -0.3401963  0.2047698  -1.661  0.09664 .
AD          -1.3409002  0.4858741  -2.760  0.00578 **
AE          -0.0006299  0.0051338  -0.123  0.90234
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

(Dispersion parameter for Negative Binomial(2.7303) family taken to be 1)

Null deviance: 516.494  on 56  degrees of freedom
Residual deviance:  61.426  on 26  degrees of freedom
AIC: 790.8

Number of Fisher Scoring iterations: 1

Theta:  2.730
Std. Err.:  0.537

2 x log-likelihood:  -726.803


When not indicating maxit, or maxit=100, it shows

Warning messages: 1: glm.fit: algorithm did not converge; 2: In glm.nb(A ~ ., data = d, maxit = 100) : alternation limit reached.

When indicating maxit=1000, the warning message disappears.

Then I create a first model:

first.model <- glm.nb(A ~ 1, data = d)


Then I tried the forward stepwise regression:

step.model <- step(first.model, direction="forward", scope=formula(full.model))


Error in glm.fit(X, y, wt, offset = offset, family = object\$family, control = object$control) : NA/NaN/Inf in 'x' In addition: Warning message: step size truncated due to divergence

What is the problem?

I also tried the backward regression:

step.model2 <- step(full.model,direction="backward")

#the final step
Step:  AIC=770.45
A ~ B + C + E + F + G + H + I + K + L + R + T + V + W + Y + AA +

Df Deviance    AIC
<none>      62.375 770.45
- AB    1   64.859 770.93
- H     1   65.227 771.30
- V     1   65.240 771.31
- L     1   65.291 771.36
- Y     1   65.831 771.90
- B     1   66.051 772.12
- C     1   67.941 774.01
- AA    1   69.877 775.95
- K     1   70.411 776.48
- W     1   71.526 777.60
- I     1   71.863 777.94
- E     1   72.338 778.41
- G     1   73.344 779.42
- F     1   73.510 779.58
- R     1   80.358 786.43
- T     1   95.725 801.80
Warning messages:
1: glm.fit: algorithm did not converge
2: glm.fit: algorithm did not converge
3: glm.fit: algorithm did not converge
4: glm.fit: algorithm did not converge


My question is: Why it is different in using forward and backward stepwise regression? And why do I get the error message when performing forward selection? Also, what exactly do these warning messages mean? And how should I deal with it?

I am not a stats person but need to conduct statical analysis for my research data. So I am struggling in learning how to do different regression analyses using real data. I searched online for similar questions but I still could understand ... And please let me know if I did anything wrong in my regression analysis. I would really appreciate it if you could help me with these questions!

• Forward and backward selection like this are not expected to give the same result, but stepwise variable selection is mostly considered poor practice.
– Dave
Sep 18, 2020 at 22:44
• I read online that people are saying that stepwise selection is not a good practice in many cases, but this is the only methods I know and feel comfortable applying. Do you have any suggestions on how to perform variable-selection for the negative binomial regression? Thank you, @Dave!
– XM_Z
Sep 18, 2020 at 22:46
• Do not perform algorithmic variable selection. Instead, let theory guide your choice of variables, and report negative findings along with positive findings. Sep 19, 2020 at 0:36
• As per my answer below: can you clarify why you need/want to do variable selection? Sep 19, 2020 at 2:05
• Hi @Alexis, could you clarify "let theory guide your choice of variables"? Thank you!
– XM_Z
Sep 19, 2020 at 2:49

I have good news and bad news.

## good news

• you can probably more or less disregard the warnings. Where stepwise regression is recommended at all (see below ...), backward regression is probably better than forward regression anyway.
• you can do forward and backward stepwise regression with MASS::stepAIC() (instead of step).

• step probably isn't doing what you think it's doing anyway. Rather than refitting the negative binomial dispersion parameter, it's re-fitting with a fixed overdispersion parameter, which is probably not what you want (there's a classically snarky e-mail from Prof. Brian Ripley from 2006 here that discusses this issue in passing). As mentioned above, stepAIC() works better.
• if you are only interested in predictive accuracy, and not in anything about confidence intervals or hypothesis tests or measuring variable importance ... then stepwise regression might be OK (Murtaugh 2009) ...
• but if you care at all about being able to make any inferences about the effects of the parameters, you have too many variables and not enough data. A rule of thumb is that (1) you need at least 10 times as many data points as predictor variables to do reliable inference and (2) doing any inference after selecting variables (via stepwise selection or otherwise) is very wrong [unless you do super-cutting-edge stuff that only works with huge data sets and very strong assumptions].

The big question here is: why do you want to do variable selection in the first place?

• you're only interested in prediction: OK, but something like penalized regression (Dahlgren 2010) will probably work better
• you're interested in inference: this is going to be tough; you almost certainly don't have enough data to tell the effects of correlated variables apart. In your situation I would probably compute the principal components (PCA) of the predictor variables and use only the first 5 (which fall within the $$n/10$$ rule, and explain 99.5% of the variance in the predictors ...)

Murtaugh, Paul A. “Performance of Several Variable-Selection Methods Applied to Real Ecological Data.” Ecology Letters 12, no. 10 (October 2009): 1061–68. https://doi.org/10.1111/j.1461-0248.2009.01361.x.

Dahlgren, Johan P. “Alternative Regression Methods Are Not Considered in Murtaugh (2009) or by Ecologists in General.” Ecology Letters 13, no. 5 (May 1, 2010): E7–9. https://doi.org/10.1111/j.1461-0248.2010.01460.x.

• " only interested in predictive accuracy". Stepwise methods are as likely to retain false predictors as true predictors, and are as likely to exclude true predictors as false predictors. There is a reasonably mature literature on this specific point. Sep 19, 2020 at 0:38
• @Alexis, agree that there are better approaches for stepwise methods. Are you talking about correlated predictors (in which case we don't really care whether we select correlated predictor X1 or X2 as long as we pick one or the other)? Sep 19, 2020 at 0:59
• @BenBolker, Thank you very much for your answer! It took me some time to read the recommended articles. The reason that I want to perform variable selection is that from what I learned for linear regression when I have too many variables, I need to perform variable selection to choose the most important variables. Also, from the textbook, because after variable selection we still need to consider higher-order terms and interactions in the regression model, so I do not have enough degree of freedom if I want to include all the variables in my regression.
– XM_Z
Sep 19, 2020 at 2:44
• To answer your question above: I am interested in inference: to analyze if these variables have influence on the Y (Column A). I want to know 1) if those variables affect Y 2) how the variables affect Y. I did learn that I do not have enough data for the model building, that is why I wanted to perform a variable selection in the first place. I am learning the linear regression and generalized linear regression by myself, so I am not sure if I am on the right path. I also posted some general questions in another post.
– XM_Z
Sep 19, 2020 at 2:45
• Also, after reading your recommended article, I notice one thing in the Dahlgren(2010) “it should be emphasized that in many ecological studies we may not need to go past fitting a ‘full’ model and into model building, regardless of whether the model is for prediction or explanation”. I may understand it wrong, but here does “not need to go past fitting a full model” mean that in some ecological studies, we don’t have to fit a model with higher-order terms and interactions?
– XM_Z
Sep 19, 2020 at 2:45