求助各位前辈！！
RTs <- read.delim(file.choose(), row.names=1)
summary(RTs)
RTs$FAMILIARITY <- factor(RTs$FAMILIARITY, levels=levels(RTs$FAMILIARITY)[c(2, 3, 1)])

RTs$IMAGEABILITY <- factor(RTs$IMAGEABILITY, levels=levels(RTs$IMAGEABILITY)[c(2, 1)])


为什么改变了因子的排序方式后，输出的变量FAMILIARITY和IMAGEABILITY成了NA值呢？？

实在太费解了，希望各给前辈给解答下！万分感谢！