Help with Sample Size Needed
Six Sigma – iSixSigma › Forums › General Forums › New to Lean Six Sigma › Help with Sample Size Needed
 This topic has 9 replies, 3 voices, and was last updated 6 years, 7 months ago by John.

AuthorPosts

May 26, 2015 at 6:20 pm #55037
JohnParticipant@john.lombardo Include @john.lombardo in your post and this person will
be notified via email.Hi Everyone,
I’m new to LSS and am currently taking a course.
One of the assignments seems very straightforward, but I simply cannot wrap my head around it. Can anyone help?
There are 4 parts to this assignment, but I’ve only included the 1st part as well as my response for it.
Be kind because I am new to the stats world…
Question Part 1:
A part is not to exceed a failure rate of 3.4 failures in one million.
(a) Determine the sample size needed for a hypothesis test of the equality of this failure rate to a 3.4 parts per million failure rate. Use risks of 0.05 and an uncertainty of +/ 10% of the failure rate target.Response to Part 1:
(a) Sample size needed (n) for a failure rate criterion of 3.4 DPMO (3.4 / 1,000,000 = 0.0000034)
• Risk = a = 0.05 (or 95% confidence level)
Or Za = 1.645
• St.Dev. = +/ 10% = 1,000,000 * 0.1 = +/ 100,000
Or 1,000,000 +/ 100,000
900,000 <= p <= 1,100,000
• n = ((Ua * (pa * (1 pa))1/2) + (Ub * (pb * (1pb)) 1/2))/(pb – pa)
pa = 3.4 / 1,100,000 = 3.0909090909090909090909090909091e7
• pa = 0.0000003091
pb = 3.4 / 900,000 = 3.74777777777777777777777777777778e6
• pb = 0.00000375
Ua = 1.645 (Table B, Appendix A, SingleSided Normal Distribution)
Ub = 1.645 (Table B, Appendix A, SingleSided Normal Distribution)
n = ((1.645 * (9.091 * (1 – 0.0000003091))1/2) + (1.645 * (1.111 * (1 – 0.00000375))1/2))/(0.00000375 – 0.0000003091)
n = ((1.645 * (9.091 * 0.9999996909)1/2) + (1.645 * (1.111 * 0.99999625)1/2))/(0.0000034409)
n = ((1.645 * (9.0909971899719)1/2) + (1.645 * (1.11099583375)1/2))/(0.0000034409)
n = ((1.645 * 3.015) + (1.645 * 1.054))/(0.0000034409)
n = 1.73383/0.0000034409
n = 503888.52 = 503,8890May 26, 2015 at 6:21 pm #198319
JohnParticipant@john.lombardo Include @john.lombardo in your post and this person will
be notified via email.Sorry, the formatting came out wrong…
Also, I have access to Minitab 16 but am new to that too…
0May 27, 2015 at 7:21 am #198324
Amit Kumar OjhaParticipant@AmitOjha Include @AmitOjha in your post and this person will
be notified via email.Hi John,
Please find below the solution for the given problem:
It is given thatTolerance = 10
Alpha = 0.05 and Hence Z = 1.96
Standard Deviation corresponding to 3.4 parts per million defect rate = 6
Hence Sample Size = ((1.96*6)/10)^2 = 32.14So you need a sample of size = 33 (Rounded Off)
Good Luck !!!
0May 27, 2015 at 8:40 am #198326
JohnParticipant@john.lombardo Include @john.lombardo in your post and this person will
be notified via email.Thank you AmitOjha,
How did you determine Z?
I know 3.4 dpmo is the 6 sigma standard (really its equal to 4.5 sigma).
And that is used as the st.dev., but are you using reference tables or are these calculated?Also, is this related to a type of 1 proportion test
Regards,
John0May 27, 2015 at 5:59 pm #198329
JohnParticipant@john.lombardo Include @john.lombardo in your post and this person will
be notified via email.Ok, i realize my last question on Z is silly. I realize the ztable contains my answer.
But I am being pointed towards a 1 proportion type test to solve this for the assignment.
Can you help clarify?
0May 27, 2015 at 11:07 pm #198330
Amit Kumar OjhaParticipant@AmitOjha Include @AmitOjha in your post and this person will
be notified via email.Hi,
by 1 proportion test for determining sample size what exactly do you imply ? Sorry I dint get that :(
0May 28, 2015 at 8:16 am #198335
NigelParticipant@Nigelbloomy Include @Nigelbloomy in your post and this person will
be notified via email.@John.Lombardo Do you mean a 1 tailed test vs a 2 tailed test? Amit probably knows better but I think that would mean you need to use a Zscore of 1.645 instead of 1.96.
This article explains the difference between the 2 tests:
http://www.ats.ucla.edu/stat/mult_pkg/faq/general/tail_tests.htm0May 28, 2015 at 2:20 pm #198336
JohnParticipant@john.lombardo Include @john.lombardo in your post and this person will
be notified via email.Hi,
So the final solution that I submitted (and was accepted) is below.
For the 1 proportion test, this is done through minitab as follows:
Go to: Stat –> Power and sample size –> 1 Proportion …
Then enter into the dialoge:
Comparison propotion = 0.00000374
Power value = 0.95
Hypothesized proportion: 0.00000306Or you can do it the manual way using the following formula:
n = (((Ua * (pa * (1 pa))^1/2) + (Ub * (pb * (1pb))^1/2))/(pb – pa))^2
• pa = 0.0000034 * 1.1 = 0.00000374
• pb = 0.0000034 * 0.9 = 0.00000306
• Ua = 1.645 (Table B, Appendix A, SingleSided Normal Distribution)
• Ub = 1.645 (Table B, Appendix A, SingleSided Normal Distribution)
• n = (((1.645 * (0.00000374 * (1 – 0.00000374))1/2) + (1.645 * (0.00000306 * (1 – 0.00000306))1/2))/(0.00000306 – 0.00000374))^2
• n = 79,389,227In any case, the sample size is deemed unmanageable!
0May 28, 2015 at 11:10 pm #198339
Amit Kumar OjhaParticipant@AmitOjha Include @AmitOjha in your post and this person will
be notified via email.Hi John,
See as far as I know, there can be only one point which you need to take care about the formula for sample size calculation i.e. whether you are concerned about one tailed or two tailed test.
In this context Nigel is correct.However, the formula which you used to calculate the sample size is not correct.
Refer Levin and Rubin – Statistic for Management for sample size calculation. This is best book I have ever known for statistics for beginners.
0June 9, 2015 at 8:23 am #198391
JohnParticipant@john.lombardo Include @john.lombardo in your post and this person will
be notified via email.Thank you Amit, I will certainly take a look at it!
0 
AuthorPosts
You must be logged in to reply to this topic.