We’re often asked how we calculate our percentage change figures.
There is one simple calculation we use to work out the percentage change, this is the same whether you’re looking for week on week (WOW) change or year on year (YOY) change. The golden rule for this formula is that the dates being compared must be the same length (e.g. 1 week vs 1 week or 1 year vs 1 year) in order to give a meaningful result.
To work through a pedestrian count example, we’ll assume that this (current) month we saw 5,500 visitors. This month last year (previous) we saw 5,000 visitors. Plugging these numbers into the formula gives us:
Therefore, we have seen a 10% increase in monthly pedestrian counts compared to last year.
*Tip* If you're using Excel to perform the calculation and you have the cell formatted as %, omit the x100 part from the calculation as Excel will do this for you.
As you may remember from our blog “Tips & Tricks – Our Calendar Structure”, not all months have the same number of weeks – for example March has 5 weeks and February has 4. In this case you cannot simply insert the monthly totals into the formula as you would be trying to compare 5 weeks with 4; and what was the golden rule about this formula? Periods being compared must be the same length.
How then do we deal with this if we want to compare March to February?
To compensate for the additional week in March, we take an average week for each month and plug those values into the formula above.
March weekly average = Monthly Total / 5
February weekly average = Monthly Total / 4
*Tip* Remember, this percentage change formula can be used for any measure not just pedestrian counts – e.g. Total Sales, Days of sunshine, Reported Crimes, etc.
And that’s it, that’s how we calculate the percentage changes.
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See also the blog about our calendar structure here.