[R] logistic regression

[SNN]

Hi All,

I have data with 400 individuals and the following information 
Grade: pass or fail 
Sex: male or female 
Age 
Teaching.metho : can be  1,2,3 
TotalHours: can be 0,1,2

I want to fit a logistic regression and for the TotalHours I am getting
nothing! What could be the reason. What does the following message mean ?
 [Coefficients: (1 not defined because of singularities)] 

Below is my output

Thanks for your help



Call:
glm(formula = Grade~ sex + age + teaching.method+ TotalHours, 
    family = binomial, data = data)

Deviance Residuals: 
    Min       1Q   Median       3Q      Max  
-1.3844  -0.9686  -0.7688   1.2304   1.8871  

Coefficients: (1 not defined because of singularities)
                Estimate Std. Error z value Pr(>|z|)   
(Intercept)     -3.62178    1.20480  -3.006  0.00265 **
sex             -0.32709    0.28539  -1.146  0.25175   
age              0.04405    0.01371   3.213  0.00132 **
teaching.method   0.23878    0.20553   1.162  0.24533   
TotalHours             NA         NA      NA       NA   
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 

(Dispersion parameter for binomial family taken to be 1)

    Null deviance: 297.38  on 223  degrees of freedom
Residual deviance: 282.93  on 220  degrees of freedom
AIC: 290.93


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