But artificial manipulation of fertility through contraception or other practices would make it only useful if you managed to discount those practices.
Just because an intelligent couple choose to have no children has no necessary meaning to their true "biological fitness"?
Biological Fitness is an empiric not a moral measurement.
There are two accepted empiric measures of biological fitness these are Absolute Fitness and Relative Fitness. Both of these are directly proportional to fertility unless there is a large differences in infant mortality between the groups. As all of the data comes from one country USA there should be no large differences in infant mortality.
Now the case can certainly be made that Biological Fitness is an irrelevant metric. However, it is likely that some readers will feel this metric to be important and that makes the data relevant.