From: Martin Van der Linden
In chapter 1 page 6, you mention the case of the effect of start age on results in school as and example of fundamentally unidentified questions. Do you mean that what cannot be assessed experimentally is the very effect of starting school later because a student who starts school at 6 and is perfectly identical in all dimensions but start age to another student starting school at 7 cannot be found?
If true, is there any reason we would like to measure this pure effect of start age independently of maturation effect? Isn't it precisely maturation effect we try to measure when thinking about start age? yes! any first grader who started at 7 will be older than a first grader who starts at age 6 on test day. Since we think there are big age-at-test effects, the comparison in test scores between these two is misleading Many school districts would like to boost their test scores and are tempted to go for an older start age to do it. Older start ages will indeed boost scores (suppose you couldn't enter first grade until after your bar mitzvah ...) But that fact doesn't mean older entrants are learning more; they might well do worse (e.g., by virtue of the dropout age mechanism detailed in Angrist and Krueger 1991) JA