问题
你想要改变因子水平出现的次序。
方案
R中有两种不同类型的因子变量：有序和无序。比如{小，中，大}和｛钢笔，橡皮擦，铅笔｝。对于绝大多数分析而言，一个因子变量是有序还是无序的不重要。如果因子是有序的，那么这个因子水平的特定次序是重要的（小<中<大）。如果因子是无序的，那么因子水平同样会以一定的顺序出现，但这仅仅为了方便而已（钢笔，橡皮擦，铅笔） - 但有时它是重要的，例如它会觉得结果如何输出，图形元素如何展示。
一种改变因子次序的方式是对因子使用factor()函数并且直接指定它们的次序。下面这个例子中，ordered()函数可以替换factor()函数。
下面是这个例子的数据：
# 创建一个错误次序的因子
sizes <- factor(c("small", "large", "large", "small", "medium"))
sizes
#> [1] small large large small medium
#> Levels: large medium small
因子水平被显式地指定：
sizes <- factor(sizes, levels = c("small", "medium", "large"))
sizes
#> [1] small large large small medium
#> Levels: small medium large
我们同样可以对有序因子这样操作：
sizes <- ordered(c("small", "large", "large", "small", "medium"))
sizes <- ordered(sizes, levels = c("small", "medium", "large"))
sizes
#> [1] small large large small medium
#> Levels: small < medium < large
另一种方式是使用relevel()函数在列表中制作一个特定水平（这对有序因子不起作用）。
# 创建错误次序的因子
sizes <- factor(c("small", "large", "large", "small", "medium"))
sizes
#> [1] small large large small medium
#> Levels: large medium small
# 使得 medium 排最前面
sizes <- relevel(sizes, "medium")
sizes
#> [1] small large large small medium
#> Levels: medium large small
# 使得 small 排最前面
sizes <- relevel(sizes, "small")
sizes
#> [1] small large large small medium
#> Levels: small medium large
当因子创建时，我们可以指定合适的顺序。
sizes <- factor(c("small", "large", "large", "small", "medium"),
levels = c("small", "medium", "large"))
sizes
#> [1] small large large small medium
#> Levels: small medium large
反转因子水平次序。
# 创建错误次序的因子
sizes <- factor(c("small", "large", "large", "small", "medium"))
sizes
#> [1] small large large small medium
#> Levels: large medium small
sizes <- factor(sizes, levels=rev(levels(sizes)))
sizes
#> [1] small large large small medium
#> Levels: small medium large
原文链接：http://www.cookbook-r.com/Manipulating_data/Changing_the_order_of_levels_of_a_factor/
