Consider the logistic regression model, Model 1, given by

log(pi / [1-pi]) = 0.25 + 0.32*X1 + 0.70*X2 + 0.50*X3

where X3 is an indicator variable with X3=0 if the observation is from Group A and X3=1 if the observation is from Group B.  The likelihood value for this fitted model on 100 observations is 0.0850.

a) For X1=2 and X2=1 compute the log-odds for each group, i.e. X3=0 and X3=1.

b) For X1=2 and X2=1 compute the odds for each group, i.e. X3=0 and X3=1.

c) For X1=2 and X2=1 compute the probability of an event for each group, i.e. X3=0 and X3=1.

d) Using the equation for M1, compute the relative odds associated with X3, i.e. the relative odds of Group B compared to Group A.

e) Use the odds for each group to compute the relative odds of Group B to Group A.   How does this number compare to the result in Question #7.  Does this make sense?

Model 2:  Now let’s consider an alternate logistic regression model, which we will refer to as Model 2, given by

log(π/ [1-π]) = 0.25 + 0.32*X1 + 0.70*X2 + 0.50*X3 + 0.1*X4

where X3 is an indicator variable with X3=0 if the observation is from Group A and X3=1 if the observation is from Group B.  The likelihood value from fitting this model to the same 100 observations as M1 is 0.0910.

f) Use the G statistic to perform a likelihood ratio test of nested models for M1 and M2.  State the hypothesis that is being tested, compute the test statistic, and test the statistical significance using a critical value for alpha=0.05.  From these results should we prefer M1 or M2?

asked May 19, 2014