# The ARL for an X bar Control Chart (n = 5)

#### leaning

##### Involved In Discussions
Just to verify:
The ARL for an X bar control chart(n = 5) assuming the process remains in control is 370, right?

(The answer key has 8 has the right answer, but this practice test is notorious for the answers being wrong.)

Regards,
leaning

#### leaning

##### Involved In Discussions
Miner,

Ha! That same document was what I looked at for 370. It matched some other 370's I found Googling books. it's the practice tests that are killing me because they call the wrong answer correct. (For this question, they say the correct answer is 8.). Grrr!

Thanks for the reply and validation.

Regards,
leaning

#### Miner

##### Forum Moderator
Staff member
I am far from an expert in ARL. I did find the following formula. Using alpha = 0.0027 (1 - .9973 = 0.0027), 1/0.0027 = 370.37, which rounds to 370.

What is the exact wording of the question? 8 points on one side of the mean is usually used as a signal of an out of control run, which is not the same as ARL.

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
ARL is an old concept that is more theoretically pure than of practical importance or value.

what class or test are you studying for?

#### Miner

##### Forum Moderator
Staff member
ARL is an old concept that is more theoretically pure than of practical importance or value.

what class or test are you studying for?

Definitely an old concept with no real value. I suspect this is for the CQE exam.

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