What if the patient has several chronic diseases that could have been underlying causes of death? Well, you only get to pick one, and only that one gets recorded as the cause of death. You can list the other diseases too, but not as causes of death.

It seems I miscommunicated here. What I meant to say that listing these other diseases has no meaningful impact on the mortality statistics, although technically speaking they are causes of death. If the point is to gather accurate statistics, listing them feels like a consolation prize, because statisticians don't seem to be interested in them.

In Finland a direct translation for these would be "contributory causes of death". That's probably the same thing as secondary causes of death. The problem is, it's difficult for someone who makes these into statistics to know how important they were. Almost anything the patient has can be listed as a contributory cause of death.

Even a bigger problem is that listing them is completely optional. If almost nobody fills them in properly (because they usually have better things to do), that is another good reason for a statistician not to use them.

Is filling in the secondary causes mandatory in US? Are there clear restrictions for what can be listed? If not, I'm not sure if they provide all that useful information, statistically speaking. Are they really used in meaningful way in any statistics?

Irregularities usually appear when there is a switch from one ICD-Code to a new one, so in 1969,1979,1999.

I suppose WHO recommendations for filling these certificates impact the US too.