查看 plyr 教程,我发现以下准备工作:
b2 <- ddply(baseball, "id", transform, cyear = year - min(year) + 1)
b2 <- ddply(b2, "id", transform, career = (cyear - 1) / max(cyear))
bruth <- subset(b2, id == "ruthba01")
# Could we model that as two straight lines?
bruth$p <- (bruth$career - 0.5) * 100
现在一些模型
mod <- lm(g ~ p + p:I(p > 0), data = bruth)
有什么区别?
mod <- lm(g ~ p + I(p > 0), data = bruth)
当我检查
mod$model
在这两种情况下,它都会产生具有相同数字的相同列。
然而回归系数完全不同......
知道这个符号是什么意思吗?