Creating a measure that combines a percentage with a low decimal number?

Question

I'm working on a project in Tableau (which uses functions very similar to Excel, if that helps) where I need a single measurement derived from two different measurements, one of which is a low decimal number (2.95 on the high end, 0.00667 on the low) and the other is a percentage (ranging from 29.8 to 100 percent).

Put another way, I have two tables detailing bus punctuality -- one is for high frequency routes and measured in Excess Waiting Time (EWT, in minutes), the other for low frequency routes and measured in terms of percent on time. I have a map of all the routes, and want to colour the lines based on how punctual that route is (thinner lines for routes with a low EWT or a high percentage on time; thicker lines for routes with high EWT or low percentage on time). In preparation for this, I've combined both tables and zero'd out the non-existent value.

I thought I'd do something like log(EWT + PercentOnTime), but am realizing that might not give the value I'm wanting (especially because I ultimately need an inverse of one or the other, since low EWT is favourable and high % on time favourable).

Any idea how I'd do this? Thanks!

Answer 1

If you are combining/comparing the metrics in an even manner and the data is relatively linear then all you need to do is normalise them.

If you have the EWT expected ranges (eg. 0.00667 to 2.95). Then a 2 would be

(2 - 0.00667)/(2.95 - 0.00667) = 0.67723 but because EWT is the inverse semantically to punctuality we need to use 1-0.67723 = 0.32277.

If you do the same for the Punctuality percentage range:

Eg. 80%

(80 - 29.8)/(100-29.8) = 0.7169

You can compare these metrics because they are normalised (between 0-1 : multiply by 100 to get percentages) if you are assuming the underlying metrics (1-EWT) and on time percentage (OTP) are analogous.

Thus you can combine these into a single table. You will want to ignore all zero'd values as this is actually an indication you have no data at these points.

you'll have to use an if statement to say something like :

if OTP > 0 then ([OTP-29.8])/(100-29.8) else (1-(([EWT]-0.00667)/(2.95-0.00667)))

Hope this helps.