Research on USPTO and espacenet

I have done the same research with two strategies
In the first one, I have made the research on espacenet and then used the "addpatents" option on USPTO with the same request
If I look to the analysis by publication date, i obtain the same results roughly 246 families
Remark : on espacenet only , I get 237 families only
However when looking at the priority date
In the first case, nearly all the family have a priority date, only one US and two japanese don't
In the second case, more than 50 Us and also 2 japanse have no priority date
I have also remarked that, for example on patent, WO9918161 A1 that has 3 other patents in the family US6166123 A; US6162856 A; AU9686998 A
In matheo patent , for both research one of the patent is not linked to the WO patent US6162856 A
For the WO9806768 A1 patent espacenet gives 8 other patents in the family including US6031045 A ; US5807919 A ; AU4069397 A
Matheo gives two families :
WO9806768 A1 with priority to US6031045 A
AU4069397 A with priority to US5807919 A

My questions are :
Why are they such differences on priority date et patent? these informations are very important for statistics (I suggest that in a next version, we could obtain graph on the priority date by year as for the publication date)
What is the best strategy?
Is it necessary to make the research on both espacenet and USPTO ?
Question

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Best regards

Hi,

Here http://www.matheo-software.com/download/Matheo-Patent-6_1.exe , you can find a beta-test version rev. 050111 which includes :

- Adding PR field in statistics
- Pictures in abstract
- Desc & Claims available in export TXT & XML

Sincerely,

Bruno