"Teams’ actual choices are dramatically more conservative than those recommended by the dynamic-programming analysis. On the 1,604 fourth downs in the sample for which the analysis implies that teams are on average better off kicking, they went for it only nine times. But on the 1,068 fourth downs for which the analysis implies that teams are on average better off going for it, they kicked 959 times."
http://elsa.berkeley.edu/~dromer/papers/JPE_April06.pdf
"Teams’ actual choices are dramatically more conservative than those recommended by the dynamic-programming analysis. On the 1,604 fourth downs in the sample for which the analysis implies that teams are on average better off kicking, they went for it only nine times. But on the 1,068 fourth downs for which the analysis implies that teams are on average better off going for it, they kicked 959 times."