The document discusses optimizing year-on-year comparisons in Power BI by addressing the limitations of the sameperiodlastyear function, especially during leap years. It presents a solution that involves creating a new measure that compares data from 364 days prior, ensuring weekly alignment for accurate analysis. This technique is particularly useful for datasets that fluctuate between weekdays and weekends.

1. Power BI Tips and Timesavers
Comparing YEAR on YEAR
in Power BI

2. Power BI has a built in function SAMEPERIODLASTYEAR
But, it does not necessarily give you what you need, here we show an
alternative to make sure year on year comparisons match by weekday

3. Why would you need to
SAMEPERIODLASTYEAR does not take the day of the week into account
And this year is a leap year, so the dates are two days out of sync

4. The Simple Solution
We will use a matching date from 364 days earlier for comparison. Then each
weekday will always match even across a leap year

5. Use Case Example
Data that varies between weekdays and weekends need to have sensible
comparisons, here we can see website impression which drop at the weekends

6. Step by Step Solution
The first thing we will do is show what happens when we use
the same period last year function
We create a measure to return the Impressions for the
previous year using the SAMEPERIODLASTYEAR function
Impressions LY =
CALCULATE(sum('Sample GSC Data'[Impressions]),
SAMEPERIODLASTYEAR('Sample GSC Data'[Date]))
Then we can easily add the result into a simple table to see
the result

7. Step by Step Solution
At first glance this looks OK
But validation the data back to prior year we can that
SAMEPERIODLASTYEAR is not working as we would like it to
The standout number for last year is the 13 showing against the
13th of April being compared to Saturday the 13th of April 2024
Looking back at the 13th of April 2023 we can see it was a Thursday

8. Step 2 – Create a Measure to return the correct
Year on Year comparison
We know that if we compare to 364 days ago we will always match the weekday. So we create a filtered
measure that does this
Impressions LY2
= CALCULATE(sum('Sample GSC Data'[Impressions]), DATEADD('Sample GSC Data'[Date],-364,DAY))
We use the DATEADD function, with a minus 364 days value to match to the correct dates
it now correctly aligns to the week day

9. If you need to have comparison of year on year performance where the
data is by day, then you need to use this technique or similar
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Timesavers, visit our website
Business Analytics Blog – Select Distinct
Credit: simon.harrison@selectdistinct.co.uk