Confounding by indication

One of the many quotes attributed to film producer Samuel Goldwyn is

Any man who goes to a psychiatrist ought to have his head examined.

This neatly summarises what epidemiologists call ‘confounding by indication’, that is, the fact that treatments tend to look harmful just because they are only given to sick people.  For example, in a study I worked on in Seattle, elderly people who were taking blood-pressure medication were about 20% more likely to have heart attackes than those not taking blood-pressure medication, although we know from randomised trials that these blood-pressure medications actually reduce heart attacks by about 20%.

The impact can easily be much more dramatic: about 12% of people receiving a heart transplant die within a year, compared to 0.6% of all Kiwis, a 20-fold higher rate, and the HIV deniers have made much of the fact that nearly  all AIDS deaths in Western countries are in people who have received antiretroviral treatment for HIV infection.

In the Herald, Rodney Hide doesn’t seem to appreciate the power of confounding by indication.

The benefit-supported children were six times more likely to be abused than those who were not benefit-supported. And they were 14 times more likely to be known to Youth Justice.

Those in households benefit-dependent for nine or more years were 13 times more likely to be abused and 29 times more likely to be known to Youth Justice.

He concedes that the numbers don’t prove that the benefit support is the cause, and describes some of the factors that might lead to confounding by indication, but says

Nonetheless, the ministry factsheet is suggestive. If the benefit system were a commercial product the Government would demand a warning: Danger: Taking a benefit can endanger your children.

and describes the Ministry’s

“These findings are consistent with associations between low income and measures of child maltreatment found both across and within countries. They do not, however, establish that being supported by the benefit system causes a child to be more at risk of these outcomes.”

as doublespeak.  But in fact, the Ministry is quite right.  Comparing people who need and qualify for benefits to the rest of the country isn’t even suggestive, any more than a comparison of heart transplant recipients or people taking antiretroviral drugs to the rest of us.  And that’s without even considering the ‘PC’ issues such as whether abusing your kids might be harder to hide if you’re on benefit than if you’re rich.

Readers of history or classic literature will recall that poor children didn’t fare all that well before benefits were introducted, and a brief look at the UNICEF web site will confirm that today children can be much worse off in countries where there isn’t a functioning government benefit system.  Is that “suggestive” too?