Holding most of the control variables constant at 1990 levels fails to account for the geographic mobility of Americans.

Next, the researchers tested 48 "control variables." The purpose of these adjustments is to account for factors unrelated to USAID spending that affect mortality rates.

Holding control variables constant at 1990 levels ignores important changes that occurred on a yearly basis between 1994 to 1999.

Determining the variables to be controlled for is usually a major problem in the social sciences when analyzing possible causal relations. A structural modelling approach, having recourse to directed ...