Well, it really depends on exactly what you are trying to do.
One way that may work for your scenario is to create an 'Adhoc group' using your date field. Group all the 1970-1975 dates into one group and leave the other values alone.
Then, you can create a table calculation specifying '% difference from First' and make sure that your 1970-1975 group is first in the visualization (by manually sorting if necessary).
There are other ways to do it as well using custom SQL in the connection dialog that my be more flexible if the above does not work for your scenario.
Here is an example of what Ross was describing with US census data. I created an ad-hoc group to combine 1960-1980 data. Then I added a table calculation by right clicking on the measure. This was a 'percent difference from' calculation based on the 'first' entry.
A couple of other nifty analyses are added as well.
Population growth.twbx 31.2 KB
Thanks, guys! I think this is what I'm trying to do. I will try to replicate this in my own data to see if I can get it to work.
Some of it can also be done with data blending - you may have to use a calculated dummy field for blending
Create a dublicate of your data connection
you may have to tweek the data relationsships to only include relevante fields
Make sure not to use the year for blending
Create a calculated field with you base year/years
in your primary datasource create a calculation like:
PrimaryDatasource.Population / SecondaryDatasource.CalculatedBaseYearPopulation