independent variable: yield
two factors：nitrogen rates (low VS high) and application direction (Diag vs
RTK).
Anova 表明both nitrogen rates and tillage system and the interaction term of
these two have significant effect on yield.
mean yield: Low vs high: 11.9 vs 13.1
mean yield: Diag vs RTK: 12.4 vs 12.6
interaction: low -Diag 11.7; low-RTK 12.1; high-Diag 13.1; high-RTK 13.0
解释的时候是这样说的：rate对yield有显著影响， high 比low 产量要高出0.2.
application direction对yield 有显著影响，RTK 比diag高0.2.interaction对yield
有显著影响，when rate is low， there was a 0.4 yield increase when
application method was RTK compared with Diag.
这样解释是不是不对啊？老觉得有interaction term的时候不应该这样解释的。但是该
怎样解释呢？
