Credits
This page was developed and written by Karla Lindquist, Senior Statistician in the Division of Geriatrics at UCSF. We are very grateful to Karla for taking the time to develop this page and giving us permission to post it on our site.
Introduction
Binary outcomes in cohort studies are commonly analyzed by applying a logistic regression model to the data to obtain odds ratios for comparing groups with different sets of characteristics. Although this is often appropriate, there may be situations in which it is more desirable to estimate a relative risk or risk ratio (RR) instead of an odds ratio (OR). Several articles in recent medical and public health literature point out that when the outcome event is common (incidence of 10% or more), it is often more desirable to estimate an RR since there is an increasing differential between the RR and OR with increasing incidence rates, and there is a tendency for some to interpret ORs as if they are RRs ([1]-[3]). There are some who hold the opinion that the OR should be used even when the outcome is common, however ([4]). Here the purpose is to demonstrate methods for calculating the RR, assuming that it is the appropriate thing to do. There are several options for how to estimate RRs directly in SAS, which have been demonstrated to be reliable in simulated and real data sets of various sizes and outcome incidence rates ([1],[2]). Two of these methods will be demonstrated here using hypothetical data created for this purpose. Both methods use proc genmod. One estimates the RR with a log-binomial regression model, and the other uses a Poisson regression model with a robust error variance.
Example Data: Odds ratio versus relative risk
A hypothetical data set was created to illustrate two methods of estimating relative risks using SAS. The outcome generated is called lenses, to indicate if the hypothetical study participants require corrective lenses by the time they are 30 years old. Assume all participants do not need them at a baseline assessment when they are 10 years old. Assume none of them have had serious head injuries or had brain tumors or other major health problems during the 20 years between assessments. Suppose we wanted to know if requiring corrective lenses is associated with having a gene which causes one to have a lifelong love and craving for carrots (assume not having this gene results in the opposite), and that we screened everyone for this carrot gene at baseline (carrot = 1 if they have it, = 0 if not). We also noted their gender (= 1 if female, = 2 if male), and what latitude of the continental US they lived on the longest (24 to 48 degrees north). All values (N=100) were assigned using a random number generator. The data are in https://stats.idre.ucla.edu/wp-content/uploads/2016/02/eyestudy.sas7bdat. Here s a quick description of the variables:
proc means data = eyestudy maxdec = 2; var carrot gender latitude lenses; run;The MEANS Procedure Variable N Mean Std Dev Minimum Maximum ------------------------------------------------------------------------------- carrot 100 0.51 0.50 0.00 1.00 gender 100 1.48 0.50 1.00 2.00 latitude 100 35.97 7.51 24.00 48.00 lenses 100 0.53 0.50 0.00 1.00 -------------------------------------------------------------------------------
We have an overall outcome rate of 53%. So if we want to talk about whether the carrot-loving gene, gender, or latitude is associated with the risk of requiring corrective lenses by the age of 30, then relative risk is a more appropriate measure than the odds ratio. Here is a simple crosstab of carrot and lenses, which will allow us to calculate the unadjusted OR and RR by hand.
proc freq data = eyestudy; tables carrot*lenses/nopercent nocol; run;Table of carrot by lenses carrot lenses Frequency| Row Pct | 0| 1| Total ---------+--------+--------+ 0 | 17 | 32 | 49 | 34.69 | 65.31 | ---------+--------+--------+ 1 | 30 | 21 | 51 | 58.82 | 41.18 | ---------+--------+--------+ Total 47 53 100
It is interesting that fewer people with the carrot-loving gene needed corrective lenses (especially since these are fake data!). The OR and RR for those without the carrot gene versus those with it are:
OR = (32/17)/(21/30) = 2.69
RR = (32/49)/(21/51) = 1.59
We could use either proc logistic or proc genmod to calculate the OR. Since proc genmod will be used to calculate the RR, it will also be used to calculate the OR for comparison purposes (and it gives the same results as proc logistic). Here is the logistic regression with just carrot as the predictor:
proc genmod data = eyestudy descending; class carrot; model lenses = carrot/ dist = binomial link = logit; estimate 'Beta' carrot 1 -1/ exp; run;The GENMOD Procedure Model Information Data Set EYESTUDY Distribution Binomial Link Function Logit Dependent Variable lenses Observations Used 100 Class Level Information Class Levels Values carrot 2 0 1 Response Profile Ordered Total Value lenses Frequency 1 1 53 2 0 47 PROC GENMOD is modeling the probability that lenses='1'. Parameter Information Parameter Effect carrot Prm1 Intercept Prm2 carrot 0 Prm3 carrot 1 Criteria For Assessing Goodness Of Fit Criterion DF Value Value/DF Deviance 98 132.3665 1.3507 Scaled Deviance 98 132.3665 1.3507 Pearson Chi-Square 98 100.0000 1.0204 Scaled Pearson X2 98 100.0000 1.0204 Log Likelihood -66.1832 Algorithm converged. Analysis Of Parameter Estimates Standard Wald 95% Confidence Chi- Parameter DF Estimate Error Limits Square Pr > ChiSq Intercept 1 -0.3567 0.2845 -0.9143 0.2010 1.57 0.2100 carrot 0 1 0.9892 0.4136 0.1786 1.7997 5.72 0.0168 carrot 1 0 0.0000 0.0000 0.0000 0.0000 . . Scale 0 1.0000 0.0000 1.0000 1.0000 NOTE: The scale parameter was held fixed. Contrast Estimate Results Standard Chi- Label Estimate Error Alpha Confidence Limits Square Pr > ChiSq Beta 0.9892 0.4136 0.05 0.1786 1.7997 5.72 0.0168 Exp(Beta) 2.6891 1.1121 0.05 1.1956 6.0481
The estimate statement with the exp option gives us the same OR we calculated by hand above for those without the carrot gene versus those with it. Now this can be contrasted with the two methods of calculating the RR described below.
Relative risk estimation by log-binomial regression
With a very minor modification of the statements used above for the logistic regression, a log-binomial model can be run to get the RR instead of the OR. All that needs to be changed is the link function between the covariate(s) and outcome. Here it is specified as log instead of logit:
proc genmod data = eyestudy descending; class carrot; model lenses = carrot/ dist = binomial link = log; estimate 'Beta' carrot 1 -1/ exp; run;The GENMOD Procedure Model Information Data Set EYESTUDY Distribution Binomial Link Function Log Dependent Variable lenses Observations Used 100 Class Level Information Class Levels Values carrot 2 0 1 Response Profile Ordered Total Value lenses Frequency 1 1 53 2 0 47 PROC GENMOD is modeling the probability that lenses='1'. Parameter Information Parameter Effect carrot Prm1 Intercept Prm2 carrot 0 Prm3 carrot 1 Criteria For Assessing Goodness Of Fit Criterion DF Value Value/DF Deviance 98 132.3665 1.3507 Scaled Deviance 98 132.3665 1.3507 Pearson Chi-Square 98 100.0000 1.0204 Scaled Pearson X2 98 100.0000 1.0204 Log Likelihood -66.1832 Algorithm converged. Analysis Of Parameter Estimates Standard Wald 95% Confidence Chi- Parameter DF Estimate Error Limits Square Pr > ChiSq Intercept 1 -0.8873 0.1674 -1.2153 -0.5593 28.11 <.0001 carrot 0 1 0.4612 0.1971 0.0749 0.8476 5.48 0.0193 carrot 1 0 0.0000 0.0000 0.0000 0.0000 . . Scale 0 1.0000 0.0000 1.0000 1.0000 NOTE: The scale parameter was held fixed. Contrast Estimate Results Standard Chi- Label Estimate Error Alpha Confidence Limits Square Pr > ChiSq Beta 0.4612 0.1971 0.05 0.0749 0.8476 5.48 0.0193 Exp(Beta) 1.5860 0.3126 0.05 1.0778 2.3339
Now the exp option on the estimate statement gives us the estimated RR instead of the OR, and it also matches what was calculated by hand above for the RR. Notice that the standard error (SE) for the beta estimate calculated here is much smaller than that calculated in the logistic regression above (SE = 0.414), but so is the estimate itself (logistic regression beta estimate = 0.989), so the significance level is very similar (logistic regression p = 0.017) in this case. One of the criticisms of using the log-binomial model for the RR is that it produces confidence intervals that are narrower than they should be, and another is that there can be convergence problems (1,2). This is why the second approach is also presented here.
Relative risk estimation by Poisson regression with robust error variance
Zou ([2]) suggests using a modified Poisson approach to estimate the relative risk and confidence intervals by using robust error variances. Using a Poisson model without robust error variances will result in a confidence interval that is too wide. The robust error variances can be estimated by using the repeated statement and the subject identifier (here id), even if there is only one observation per subject, as Zou cleverly points out. Here is how it is done:
proc genmod data = eyestudy; class carrot id; model lenses = carrot/ dist = poisson link = log; repeated subject = id/ type = unstr; estimate 'Beta' carrot 1 -1/ exp; run;
Notice that id, the individual subject identifier, has been added to the class statement and is also on the repeated statement (with an unstructured correlation matrix), telling proc genmod to calculate the robust errors. Also notice that the distribution has been changed to Poisson, but the link function remains log.
The GENMOD Procedure
Model Information
Data Set EYESTUDY
Distribution Poisson
Link Function Log
Dependent Variable lenses
Observations Used 100
Class Level Information
Class Levels Values
carrot 2 0 1
id 100 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
...
Parameter Information
Parameter Effect carrot
Prm1 Intercept
Prm2 carrot 0
Prm3 carrot 1
Criteria For Assessing Goodness Of Fit
Criterion DF Value Value/DF
Deviance 98 64.5361 0.6585
Scaled Deviance 98 64.5361 0.6585
Pearson Chi-Square 98 47.0000 0.4796
Scaled Pearson X2 98 47.0000 0.4796
Log Likelihood -85.2681
Algorithm converged.
Analysis Of Initial Parameter Estimates
Standard Wald 95% Confidence Chi-
Parameter DF Estimate Error Limits Square Pr > ChiSq
Intercept 1 -0.8873 0.2182 -1.3150 -0.4596 16.53 <.0001
carrot 0 1 0.4612 0.2808 -0.0892 1.0116 2.70 0.1005
carrot 1 0 0.0000 0.0000 0.0000 0.0000 . .
Scale 0 1.0000 0.0000 1.0000 1.0000
NOTE: The scale parameter was held fixed.
GEE Model Information
Correlation Structure Unstructured
Subject Effect id (100 levels)
Number of Clusters 100
Correlation Matrix Dimension 1
Maximum Cluster Size 1
Minimum Cluster Size 1
Algorithm converged.
Analysis Of GEE Parameter Estimates
Empirical Standard Error Estimates
Standard 95% Confidence
Parameter Estimate Error Limits Z Pr > |Z|
Intercept -0.8873 0.1674 -1.2153 -0.5593 -5.30 <.0001
carrot 0 0.4612 0.1971 0.0749 0.8476 2.34 0.0193
carrot 1 0.0000 0.0000 0.0000 0.0000 . .
Contrast Estimate Results
Standard Chi-
Label Estimate Error Alpha Confidence Limits Square Pr > ChiSq
Beta 0.4612 0.1971 0.05 0.0749 0.8476 5.48 0.0193
Exp(Beta) 1.5860 0.3126 0.05 1.0778 2.3339
Again, the exp option on the estimate statement gives us the estimated RR, and it matches exactly what was calculated by the log-binomial method. In this case, the SE for the beta estimate and the p-value are also exactly the same as in the log-binomial model. This may not always be the case, but they should be similar. The SE calculated without the repeated statement (i.e., not using robust error variances) is 0.281, and the p-value is 0.101, so the robust method is quite different.
Adjusting the relative risk for continuous or categorical covariates
Adjusting the RR for other predictors or potential confounders is simply done by adding them to the model statement as you would in any other procedure. Here gender and latitude will be added to the model:
proc genmod data = eyestudy;
class carrot gender id;
model lenses = carrot gender latitude/ dist = poisson link = log;
repeated subject = id/ type = unstr;
estimate 'Beta Carrot' carrot 1 -1/ exp;
estimate 'Beta Gender' gender 1 -1/ exp;
estimate 'Beta Latitude' latitude 1 / exp;
run;
The GENMOD Procedure
Model Information
Data Set EYESTUDY
Distribution Poisson
Link Function Log
Dependent Variable lenses
Observations Used 100
Class Level Information
Class Levels Values
carrot 2 0 1
gender 2 1 2
id 100 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
...
Parameter Information
Parameter Effect carrot gender
Prm1 Intercept
Prm2 carrot 0
Prm3 carrot 1
Prm4 gender 1
Prm5 gender 2
Prm6 latitude
Criteria For Assessing Goodness Of Fit
Criterion DF Value Value/DF
Deviance 96 63.7618 0.6642
Scaled Deviance 96 63.7618 0.6642
Pearson Chi-Square 96 46.7434 0.4869
Scaled Pearson X2 96 46.7434 0.4869
Log Likelihood -84.8809
Algorithm converged.
Analysis Of Initial Parameter Estimates
Standard Wald 95% Confidence Chi-
Parameter DF Estimate Error Limits Square Pr > ChiSq
Intercept 1 -0.6521 0.6982 -2.0206 0.7163 0.87 0.3503
carrot 0 1 0.4832 0.2831 -0.0716 1.0381 2.91 0.0878
carrot 1 0 0.0000 0.0000 0.0000 0.0000 . .
gender 1 1 0.2052 0.2781 -0.3398 0.7502 0.54 0.4605
gender 2 0 0.0000 0.0000 0.0000 0.0000 . .
latitude 1 -0.0100 0.0190 -0.0472 0.0272 0.28 0.5980
Scale 0 1.0000 0.0000 1.0000 1.0000
NOTE: The scale parameter was held fixed.
GEE Model Information
Correlation Structure Unstructured
Subject Effect id (100 levels)
Number of Clusters 100
Correlation Matrix Dimension 1
Maximum Cluster Size 1
Minimum Cluster Size 1
Algorithm converged.
Analysis Of GEE Parameter Estimates
Empirical Standard Error Estimates
Standard 95% Confidence
Parameter Estimate Error Limits Z Pr > |Z|
Intercept -0.6521 0.4904 -1.6134 0.3091 -1.33 0.1836
carrot 0 0.4832 0.1954 0.1003 0.8662 2.47 0.0134
carrot 1 0.0000 0.0000 0.0000 0.0000 . .
gender 1 0.2052 0.1848 -0.1570 0.5674 1.11 0.2669
gender 2 0.0000 0.0000 0.0000 0.0000 . .
latitude -0.0100 0.0127 -0.0350 0.0150 -0.79 0.4324
Contrast Estimate Results
Standard Chi-
Label Estimate Error Alpha Confidence Limits Square Pr > ChiSq
Beta Carrot 0.4832 0.1954 0.05 0.1003 0.8662 6.12 0.0134
Exp(Beta Carrot) 1.6213 0.3168 0.05 1.1055 2.3777
Beta Gender 0.2052 0.1848 0.05 -0.1570 0.5674 1.23 0.2669
Exp(Beta Gender) 1.2278 0.2269 0.05 0.8547 1.7637
Beta Latitude -0.0100 0.0127 0.05 -0.0350 0.0150 0.62 0.4324
Exp(Beta Latitude) 0.9900 0.0126 0.05 0.9656 1.0151
We have also requested the RRs for gender and latitude in the estimate statement. In this case, adjusting for them does not reduce the association between having the carrot-loving gene and risk of needing corrective lenses by age 30.
One should always pay attention to goodness of fit statistics and perform other diagnostic tests. Refer to Categorical Data Analysis Using the SAS System, by M. Stokes, C. Davis and G. Kock for standard methods of checking whichever type of model you use.
References
1. McNutt LA, Wu C, Xue X, Hafner JP. Estimating the Relative Risk in Cohort Studies and Clinical Trials of Common Outcomes. Am J Epidemiol 2003; 157(10):940-3. 2. Zou G. A Modified Poisson Regression Approach to Prospective Studies with Binary Data. Am J Epidemiol 2004; 159(7):702-6. 3. Sander Greenland , Model-based Estimation of Relative Risks and Other Epidemiologic Measures in Studies of Common Outcomes and in Case-Control Studies, American Journal of Epidemiology 2004;160:301-305 4. Cook TD. Up with odds ratios! A case for odds ratios when outcomes are common. Acad Emerg Med 2002; 9:1430-4. 5. Spiegelman, D. und Hertzmark, Easy SAS Calculations for Risk or Prevalence Ratios and Differences, E American Journal of Epidemiology, 2005, 162, 199-205.