What would be a "reasonable" minimal number of observations to look for a trend over time with a linear regression? what about fitting a quadratic model?
I work with composite indices of inequality in health (SII,RII), and have only 4 waves of the survey, so 4 points (1997,2001,2004,2008).
I am not statistician, but I have the intuitive impression 4 points are not sufficient. Do you have an answer, and/or references ?
Thanks a lot,