Correct

多半 vs 大半

/ duōbàn vs dàbàn / 
Strokes

Definition

多半 and 大半 both Adverb or Noun, which means “probably; majority".

Example

多半 and 大半 as Adverbs both mean "most probably", and usually do not precede the subject of a sentence.
Here, the two words 多半 and 大半 are interchangeable.

In terms of frequency of usage; however,多半 is more frequently used.
1
Tā
他
duōbàn
多半
  
(
dàbàn
大半
  
)
shì
是
hēzuì
喝醉
le
了
  
。

He was probably drunk.

2
Míngtiān
明天
duōbàn
多半
  
(
dàbàn
大半
  
)
huì
会
qíng
晴
de
的
  
。

It is probably going to be clear tomorrow.

3
Tā
她
wánchéng
完成
xuéyè
学业
  
,
duōbàn
多半
  
(
dàbàn
大半
  
)
shì
是
kào
靠
gēge
哥哥
de
的
zīzhù
资助
  
。

She finished her school mainly with her brother's help.

多半 and 大半 as Nouns, mean "the greater part", and 一 can be added.
In examples ④ and ⑤, 多半 and 大半 are used as adverbials, the same as that in examples ① ② and ③, but the meaning is different.
4
Zhè
这
běn
本
xiǎoshuō
小说
hěn
很
xīyǐn
吸引
rén
人
  
,
wǒ
我
yī
一
xiàzi
下子
jiù
就
kàn
看
le
了
yī
一
dàbàn
大半
  
(
yīduōbàn
一多半
  
)
  
。

The novel was appealing to me and I read much of it at one sitting.

5
Lái
来
cānjiā
参加
dàhuì
大会
de
的
rén
人
  
,
duōbàn
多半
  
(
dàbàn
大半
  
)
shì
是
xuésheng
学生
  
。

Most of the meeting participants are students.

6
Wǒmen
我们
bān
班
de
的
tóngxué
同学
duōbàn
多半
  
(
dàbàn
大半
  
)
shì
是
nánfāng
南方
rén
人
  
。

Most of the students in our class are from the south.