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It's a matter of statistical probability. If you have a certain amount of sales each day for 6 months and then it just stops after a certain calendar date then there is a good probability there has been a change somewhere. It's not definite proof of anything, but it warrants investigation.
Sure there is a big influence from tubes etc but this is more of a gradual factor in pushing up ratios overall. It isn't like some tube opens up on May 5 and kills everyone's business outright.
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