Why "valid semivariogram model couldn't be calculated"?

Discussion created by bittermelonyuko on Oct 4, 2011
Latest reply on Oct 6, 2011 by EKrause-esristaff

I have been examining my different semivariogram models to get parameter estimates for my data set using the geostat wizard, ArcGIS 9.3.

A "valid semivariogram model couldn't be calculated" per geostat wizard, when I choose trend as 'constant' using universal kriging. 
However it does give me semivariogram and estimated parameters if I specify '1st order'. 
Similarly, it could not be calculated using ordinary kriging unless I specify to remove 1st order trend.

Under what situation, won't the wizard calculate semivariance and estimate parameters when no or constant trend is specified in the semivariogram modeling?
Does it prevent estimation if there's a trend and 'constant' trend is not adequate for data?

Thanks in advance!