Consider a dichotomous risk factor variable X that takes the value 1 if the risk factor is present and 0 if the risk factor is absent. The loghazard function is
where is the baseline hazard function.
The hazard ratio is defined as the ratio of the hazard for those who have the risk factor (X = 1) to the hazard for those who do not have the risk factor (X = 0). The log of the hazard ratio is
In general, the hazard ratio can be computed by exponentiating the difference of the loghazard between any two population profiles. This is the approach taken by the HAZARDRATIO statement , so the computations are available regardless of parameterization, interactions, and nestings. However, as shown in the preceding equation for , hazard ratios of main effects can be computed as functions of the parameter estimates. The remainder of this section is concerned with this methodology.
The parameter that is associated with X represents the change in the loghazard from X = 0 to X = 1. So the hazard ratio is obtained by simply exponentiating the value of the parameter that is associated with the risk factor. The hazard ratio indicates how the hazard changes as you change X from 0 to 1. For example, means that the hazard when X = 1 is twice the hazard when X = 0.
Suppose the values of the dichotomous risk factor are coded as constants a and b instead of 0 and 1. The hazard when becomes , and the hazard when becomes . The hazard ratio that corresponds to an increase in X from a to b is
Note that for any a and b such that . So the hazard ratio can be interpreted as the change in the hazard for any increase of one unit in the corresponding risk factor. However, the change in hazard for some amount other than one unit is often of greater interest. For example, a change of one pound in body weight might be too small to be considered important, whereas a change of 10 pounds might be more meaningful. The hazard ratio for a change in X from a to b is estimated by raising the hazard ratio estimate for a unit change in X to the power of as shown previously.
For a polytomous risk factor, the computation of hazard ratios depends on how the risk factor is parameterized. For illustration,
suppose that Cell
is a risk factor that has four categories: Adeno, Large, Small, and Squamous.
For the effect parameterization scheme (PARAM=
EFFECT) with Squamous as the reference group, the design variables for Cell
are as follows:
Design Variables 






Adeno 
1 
0 
0 
Large 
0 
1 
0 
Small 
0 
0 
1 
Squamous 
–1 
–1 
–1 
The loghazard for Adeno is
The loghazard for Squamous is
Therefore, the loghazard ratio of Adeno versus Squamous
For the reference cell parameterization scheme (PARAM=
REF) in which Squamous is the reference cell, the design variables for Cell
are as follows:
Design Variables 






Adeno 
1 
0 
0 
Large 
0 
1 
0 
Small 
0 
0 
1 
Squamous 
0 
0 
0 
The loghazard ratio of Adeno versus Squamous is
For the GLM parameterization scheme (PARAM= GLM), the design variables are as follows:
Design Variables 







Adeno 
1 
0 
0 
0 
Large 
0 
1 
0 
0 
Small 
0 
0 
1 
0 
Squamous 
0 
0 
0 
1 
The loghazard ratio of Adeno versus Squamous is
Consider Cell
as the only risk factor. The computation of the hazard ratio of Adeno versus Squamous for various parameterization schemes
is shown in Table 51.7.
Table 51.7: Hazard Ratio of Adeno to Squamous
Parameter Estimates 


PARAM= 




Hazard Ratio Estimates 
EFFECT 
0.5772 
–0.2115 
0.2454 


REF 
1.8830 
0.3996 
0.8565 


GLM 
1.8830 
0.3996 
0.8565 
0.0000 

The fact that the loghazard ratio () is a linear function of the parameters enables the HAZARDRATIO statement to compute the hazard ratio of the main effect even in the presence of interactions and nest effects.
To customize hazard ratios for specific units of change for a continuous risk factor, you can use the UNITS= option in a HAZARDRATIO statement to specify a list of relevant units for each explanatory variable in the model. Estimates of these customized hazard ratios are shown in a separate table. Let be a confidence interval for . The corresponding lower and upper confidence limits for the customized hazard ratio are and , respectively for , or and , respectively for .
Let be the jth unit vector—that is, the jth entry of the vector is 1 and all other entries are 0. The hazard ratio for the explanatory variable with regression coefficient is defined as . In general, a loghazard ratio can be written as (a linear combination of the regression coefficients), and the hazard ratio is obtained by replacing with .
The hazard ratio is estimated by , where is the maximum likelihood estimate of the regression coefficients .