In a business context it is common to use ‘month’ as a time unit and compare a month to its predecessor or to the same month of last year. So it is important to at least keep in mind that months can be very differently seasonally biased. Most  business dynamics exhibit a week based seasonality with specifically strong weekdays. March 2011 for example has 23 work days – 4 more than the following month (holidays considered for Bavaria). This can have a considerable effect – especially when coincidentally leveraged by the usual volatility.

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