I had a bout with bladder cancer, and got treated for it from late last year to about six weeks ago, when I returned to full-time work. It was fairly serious, and did involve major surgery as well as rebuilding a new bladder. I also had four cycles of chemotherapy, which is now done before surgery. As my oncologist explained, chemo can't really cure bladder cancer, but it helps by shrinking the tumor and killing off anything that may have metastasized. He said they do it before surgery because they can use the big tumors in the bladder to monitor how effective the chemo is on the tiny, almost invisible cancer cells that have spread.
Chemotherapy is a nasty, but often effective treatment. The best drugs for your own treatment are those that are effective against cancer, but not very effective against you. Sometimes, it doesn't work, and the chemo does more damage to the patient than to the cancer. Then it's time to change drugs or stop the chemo. And that leads to the "Chemotherapy Principle": The treatment must do more good than harm. More below the squiggle.
All chemo drugs are poisons, that can attack both cancer cells and your own normal cells. The most effective chemotherapy drugs target rapidly-dividing cells, like cancer. They will also affect any other normal, rapidly-dividing cell in your body, your stomach lining, your hair, your fingernails and your bone marrow. At best, the chemo will poison the cancer cells and have only minor effects on you - a little hair loss, a little anemia or a little nausea. At its worst, chemo drugs can do serious damage, and sometimes permanent damage without doing much to the cancer. The key is to monitor both the tumors and the side effects, and to change drugs or stop them altogether if they do more harm than good.
Voter ID screening is a lot like chemo. ANY voter screening tool, no matter how well intentioned, should have two effects. First, it should deter illegitimate voters from voting in person. Effective or not, it will also also keep legitimate voters from voting, and make it more of a hassle for more voters. An effective voter ID law will deter a lot of illegitimate voters while cauing only a little extra hassle. A nuisance law will cause a lot of hassle, and not prevent any illegitimate voting.
The "Chemotherapy Rule" as applied to voter ID will therefore be: "No voter ID law may disenfranchise more legitimate voters than the number of illegitimate voters it stops from voting. No voter ID law may cause undue hardship on more legitimate voters than the number of illegitimate voters it stops from voting."
So the exercise is simple - determine how much in-person voter fraud exists, and what fraction of that fraud the proposed voter ID law will stop. This will give you a number. The voter ID system cannot disenfranchise or unduly burden more people than that value.
At this point, most readers are likely to berate me, saying "But the states haven't proved that there is in-person voter fraud. You are absolutely right. And that leads to the main corrolary of he chemotherapy rule:
Instituting a voter ID law when there's no voter fraud is like enduring chemotherapy when you don't have cancer!
And nobody can be that stupid - except for Republicans who want to make Democrats ill.