# Average and standard deviation of Cumulative Data

##### Registered
If i calculate average of cumulative data or std seviation for a period (for example 2 year complaints ratio every month) is it proper or not? can u make some comments?

Thank you

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
What analysis are you doing? in other words what do you want to understand form the data? let's start there as that is what drives the correct analysis.

##### Registered
What analysis are you doing? in other words what do you want to understand form the data? let's start there as that is what drives the correct analysis.
Hi Bev D

Some friend from the company want to decide new kpı value from last year monthy kpı values which are calculated cumulative. They took average and std. dev. of the monthly kpı values to calculate new years target as average - 1 std dev.

#### John Predmore

Trusted Information Resource
A word of caution: The average of 24 monthly ratios is not mathematically equal to the ratio of the cumulative total numerator divided by the cumulative total denominator, unless the denominator for every month is of equal size. You can convince yourself the average of the ratios is not equal to the ratio of the averages (except in limited cases) by experimenting with columns of data in an electronic spreadsheet. When you speak of averaging standard deviations, you must be even more cautious of the calculations and what they represent.

If you ask which calculation is correct, you have to do what BevD explained, decide which analysis you desire to complete.

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
It would actually be more helpful if you could post the actual monthly data with both numerators and denominators as well as the average and SD calculations. Manipulated data hides many sins...

#### Mike S.

##### Happy to be Alive
Trusted Information Resource
Bev and John are right, IMO.

Also, does anyone even know if the process described by the kpi is in control or not?

On what basis did someone select the new target as prior average - 1 std dev? That sounds awfully arbitrary. Is anything being done to help make that goal achievable?

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