A real measure would be "how long a shipment stays on the shelves before it sells out". This is a nice analog measure, so you could graph it and get a good feeling for it.
As a few people have mentioned, there are a few factors in play here. Either way re: the thread title "Nintendo base crumbling" - no, 100% definitely not...
1/ Holiday period over
2/ No/minimal new Wii games
3/ Smaller shipments
4/ Kids back to school
Another factor is this: In Australia (at least), there have been various Wii bundles released - effectively a price drop. I haven't heard of any bundle offers in Japan, or any form of price drop.
Any if you are going to have a drop in sales - this is the perfect time of year for it. Within a couple of weeks the pre-Xmas title flood will start... and I'll be shocked if ever Wii in Japan isn't sold by the end of the year. WiiFit alone could trigger that - especially in Japan.
Gesta Non Verba
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