轉載和整理自，向作者表示感謝

在R語言中，因子（factor）表示的是一個編號或者一個等級，即，一個點。例如，人的個數可以是1，2，3，4……那麼因子就包括，1，2，3，4…..還有描述協變數水平時，會用到高、中、低，也是因子，因為這些都是一個點。與之區別的向量，是一個連續性的值，例如，數值中有1，1.1，1.2……可以作為數值來計算，而因子則不可以。簡單通俗來講：因子是一個點，向量是一個有方向的範圍。在R中，如果把數字作為因子，那麼在匯入資料之後，需要將向量轉換為因子（factor），而因子在整個計算過程中不再作為數值，而是一個”符號”而已。

以例項進行解釋和說明

data <- c(1
 
,2,2,3,1,2,3,3,1,2,3,3,1)  
> data
 [1] 1 2 2 3 1 2 3 3 1 2 3 3 1 

> fdata <- factor(data)
> fdata
 [1] 1 2 2 3 1 2 3 3 1 2 3 3 1
Levels: 1 2 3

> class(fdata)
[1] "factor"
> class(data)
[1] "numeric"

#factor()函式將原來的數值型的向量轉化為了factor型別。factor型別的向量中有Levels的概念。Levels就是factor中的所有元素的集合（沒有重複）。我們可以發現Levels就是factor中元素排除重複後且字元化的結果。因為Levels的元素都是character。
 


> levels(fdata)
[1] "1" "2" "3"

#我們可以在factor生成時，通過labels向量來指定levels，繼續上面的程式：

> rdata <- factor(data,labels=c("I","II","III"))
> rdata
 [1] I   II  II  III I   II  III III I   II  III III I  
Levels: I II III

> rdata <- factor(data,labels=c("e","ee","eee"))
> rdata
 [1] e   ee  ee  eee e   ee  eee eee e   ee  eee eee e  
Levels: e ee eee

#factors可以指定資料的順序
 


> mons <- c("March","April","January","November","January", "September","October","September","November","August", "January","November","November","February","May","August", "July","December","August","August","September","November", "February","April")
> mons <- factor(mons)
> mons
 [1] March     April     January   November  January  
 [6] September October   September November  August   
[11] January   November  November  February  May      
[16] August    July      December  August    August   
[21] September November  February  April    
11 Levels: April August December February ... September

> table(mons)
mons
    April    August  December  February   January 
        2         4         1         2         3 
     July     March       May  November   October 
        1         1         1         5         1 
September 
        3 

#顯然月份是有順序的，我們可以為factor指定順序
mons = factor(mons,levels=c("January","February","March","April","May","June","July","August","September","October","November","December"),ordered=TRUE)

> table(mons)
mons
  January  February     March     April       May 
        3         2         1         2         1 
     June      July    August September   October 
        0         1         4         3         1 
 November  December 
        5         1