Here are the lurking variables that i can think of:-
Amount :- It is obvious that the amount of DAP will also affect the yield
Interval :- The yield will also depend on the interval of adding DAP
Usage of pesticide :- The yield will also be affected by how much pesticide is used . If there are no pest tp eat the crop the production will increase.
Irrigation :- It might be possible that the absence or presence of water may have had some effects on crop yield.
This is all I can think. Please reply to this post and let me know what you people think.