I am trying to analyse number of errors over a period of 2 years and I run a normality test for my data via minitab.

it shows Attachment 24567

Can i conclude that the graphical representation show non-normal data ?

then what should i do ?

Thanks!

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- Aug 21st 2012, 08:50 AMhazelNormality plot (no of errors over 2 years)
I am trying to analyse number of errors over a period of 2 years and I run a normality test for my data via minitab.

it shows Attachment 24567

Can i conclude that the graphical representation show non-normal data ?

then what should i do ?

Thanks! - Aug 21st 2012, 02:55 PMpickslidesRe: Normality plot (no of errors over 2 years)
If that is a Q-Q plot then your data is not normal.

Tell us about the dataset. - Aug 22nd 2012, 08:17 AMhazelRe: Normality plot (no of errors over 2 years)
Month No. of errors occurred

Jan 0

Feb 5

Mar 1

Apr 0

. 1

. 4

.

etc...

that's all. Do you need the full data ? - Aug 29th 2012, 08:18 AMhazelRe: Normality plot (no of errors over 2 years)
Any tip from anyone ??

- Aug 29th 2012, 08:46 AMhazelRe: Normality plot (no of errors over 2 years)
i would like to test for special causes, but if my data is non-normal , is there any way out or I need to gather at least 30 for my sample size ?

i can't get 30 as I only have data from 2010. and the number of errors made is less than 30. - Aug 29th 2012, 09:19 AMMaxJasperRe: Normality plot (no of errors over 2 years)
Plot also histogram of your data. You have most of your data =0 which indicates normal dist is not a candidate and perhaps a right-skewed distribution is need like chi-square ..etc. Also check Poisson distribution. Please post a table of your complete data.

- Aug 29th 2012, 07:12 PMhazelRe: Normality plot (no of errors over 2 years)
This is my dataset :

Month No of Errors

Jan 2010 2

Feb 2010 3

Mar 2010 1

Apr 2010 1

May 2010 1

Jun 2010 4

Jul 2010 1

Aug 2010 1

Sep 2010 0

Oct 2010 2

Nov 2010 3

Dec 2010 1

Jan 2011 2

Feb 2011 2

Mar 2011 1

Apr 2011 0

May 2011 5

Jun 2011 0

Jul 2011 0

Aug 2011 0

Sep 2011 1

Oct 2011 0

Nov 2011 0

Dec 2011 1

Jan 2012 0

Feb 2012 0

Mar 2012 0

Apr 2012 4

May 2012 1

Jun 2012 1

Jul 2012 1 - Aug 29th 2012, 07:33 PMhazelRe: Normality plot (no of errors over 2 years)
I managed to get 2010 data till July 2012,so i believe the sample size (N=31) is now sufficient.

- Aug 29th 2012, 08:15 PMMaxJasperRe: Normality plot (no of errors over 2 years)
A histogram of your data indicates right-skewed data:

http://mathhelpforum.com/attachment....1&d=1346295659

Normal Q-Q diagram is:

http://mathhelpforum.com/attachment....1&d=1346295671

And variation of data form normal:

http://mathhelpforum.com/attachment....1&d=1346295677

To test whether data fits a normal distribution or a Poisson distribution:

http://mathhelpforum.com/attachment....1&d=1346296315

H0: data is normally distributed :: reject (p<0.05)

H1: data follows a Poisson distribution :: accept (p>0.05)

Data can also follow other distributions. You should check and select the best that fits and describes your data.

Please notice that at p=0.01 significance level your data can be considered marginally normally distributed but not at p=0.05 significance level. However, usually as you collect more data these conditions will change. - Aug 29th 2012, 08:17 PMMaxJasperRe: Normality plot (no of errors over 2 years)
Problem with your Q-Q is that you plotted the data points while you should plot the Expected values.

- Aug 29th 2012, 09:53 PMzzephodRe: Normality plot (no of errors over 2 years)
- Aug 30th 2012, 07:22 AMhazelRe: Normality plot (no of errors over 2 years)
- Aug 30th 2012, 06:24 PMhazelRe: Normality plot (no of errors over 2 years)
This is what I have got from minitab: Graph > Probability Plot.

Attachment 24649 - Aug 30th 2012, 06:31 PMhazelRe: Normality plot (no of errors over 2 years)
With Reference to

Identifying the Distribution of Your Data - Minitab

I do a check and I got this :

Attachment 24648

P<0.005 so it is not normally distributed. Where do i go from here ? - Sep 9th 2012, 10:36 PMhazelRe: Normality plot (no of errors over 2 years)
No one ?