Expost forecast

[Former Member]

Hi Friends,

For a product when i run univariant forecast, the system has generated "negative expost forecast values for some periods.

However in the messages the it says that since expost vaues are negative, system generated forecast is negative and it does not propose any forecast.

My question is

Can expost forecast values be negative ? If yes do we have this information in any sap help ?

Is it a standard functionality?

how to calculate a expost forecast?

Thanks

d_s

[Former Member · Accepted solution]

Hi developer_sap,

Ex-post forecast is that which is run in past periods for which actual demand history is also available. The system calculates the forecast accuracy measurements by comparing the differences between the actual values and the ex-post values.

Ex-post forecast purely depends upon the historical data values. If there is lot of fluctation in actual demand Vs actual history, then there are more chances that value comes as negative.

So, it is very critical to cleanse the past history data. If more historical values are available than the system needs to initialize the model, an ex-post forecast is carried out automatically as follows

1) The historical values are divided into two groups: the first group with the older values is used for initialization; the more recent values in the second group are used to carry out an ex-post forecast

2) The basic value, the trend value, the seasonal index, and the mean absolute deviation (MAD) are modified in every ex-post period. These values are used to calculate the forecast results in the future.

3) The system calculates the error total in the ex-post horizon. The error total is the sum of the differences between the actual (historical) values and the planned (in this instance, the ex-post forecast) values.

4) The ex-post forecast results are stored in the key figure you predefined for this purpose in Customizing.

An ex-post forecast is carried out for all forecast strategies other than 13, 14, 80 and 94. In forecast strategy 23, the alpha factor is adapted in every ex-post period based on the mean absolute deviation (MAD) and the error total (ET)

Regards

R. Senthil Mareeswaran.

[Former Member · Accepted solution]

Hi Senthil,

Thanks for your reply.

Yes i do agree that expost forecast is calculated with the historical data.

But in my case, i wanted to see how the system calculated the negative values, based on what formulae did system calculate, so that i can verify it.

Thing is, suppose i want to rectify the negative values, where should i go and make adjustements, which data exactly i ned to change? i want to know this.

Thanks

Edited by: developer_sap on May 15, 2009 12:10 PM

[Former Member]

Hi D_S

It's very simple

Expost forecast is a forecast run in the past for comparison against historical values.

current basic value + current trent value =expost forcast of next month .

Thanks!

Kriss

Edited by: kriss.m on May 14, 2009 11:43 PM

[Former Member]

Hi Kriss,

Thanks for your reply.

I did go through the formulae mentioned by SAP.

Ex-Post Forecast Ex(t +i) = G(t) + i * T(t) , where g- basic and t - trend.

But in my case, my forecast values doesnt match with this formulae.

Thanks

Edited by: developer_sap on May 15, 2009 12:09 PM

Edited by: developer_sap on May 15, 2009 12:09 PM

[Former Member]

Hi

give me your values ...i will caluclate and let you know

Thanks!

Kriss